oil pressure almost to 0 at 60mpg?!?!

Hey wassup everyone, I own a 63 Chevy Impala with a 350 motor and 350 trans. The motor was rebuilt but it seems to be having low oil pressure problems. The block is from a 79' and everything is stock . It has 600 cfm edelbrock carb and 2101 intake. I'm running 10X40 oil in it now. I am also using a Autometer mechanical oil pressure guage. When I start up the car, the oil pressure rises to about 40. When warmed up and driving around town, at a stop light and idling, I notice that it almost drops to 0. Oil pressue does go up when I start to go. When I'm driving at 55 mph it's around 25-35 but as soon as I reach 60 mph, it starts to drop to almost 0 when crusing at 60. When I reduce speed back to 55 it rises back up to around 25-35. I tried to use a heavier oil (20x50) and that only seems to help raise the pressure at idle but same problems at 60.
So far, I have done the following to no luck fixing the problem.
-New oil filter(fram P5) and oil 10X40...took about 7 quarts to fill. I am using a finned alum oil pan
-Checked the oil filter adapter to make sure bypass valve works...it does
-Checked the fitting the goes into the block to make sure teflon was not caught in passage way/cleaned out nylon tubing with canned air to make sure there is no blockage/replaced guage with a known working one.
-Engine seems to be running fine; no knocks/pings, vac is about 16, timing is 8BTDC
I have asked several people about this and most say to check the oil adapter bypass and guage which I have to no luck.
Any ideas to what my problem might be and ways to test and solve my problem? It took me about 10 yrs to build this car from the frame up and I've gone too far to let this stop me. Thanks for any advice.  

Do you remember what the bearing clearances are.  
Professional hi-performance engine builder

Horsepower sells Engines and torque wins races.

i wonder if he has a PCV hooked up... without a breather...  so the PCV system at high speed creates a lot of vacuum in the crank case...  the lowering of pressure inside .. would pull the pan upward and block the oil pump pickup screen...


its just a thought...  i would think that draining the oil... using a piece of welding rod ...  to verify the oil pump pickup is still on the pump... i have had them fall off...  i have found them too close to the pan...   i have found dog hair blocking the pickup screen...   the motor got set on a carpet with the pan off...  the carpet was full of dog hair...  

i also wonder ... if the oil drain back holes in the heads are open and free flowing..  so the oil pumps up to the valve covers and the pan goes dry...

what happens if you over fill the pan a quart or two???       oh.. and how much oil is in the pan?????   a full 5 or quarts???

is it the right dipstick????

just a few thoughts...  

I would be willing to bet your oil pump pickup fell off. most of them are a press fit and fall off when the engine warms up. summit sells ones with welded pickups for around 50 bucks.  

Or you could tack weld the pick up to the pump yourself if you have a welder.  
Professional hi-performance engine builder

Horsepower sells Engines and torque wins races.

The welds on the pumps from summit are much cleaner than most people with even moderate welding skills can produce. I read an article years ago that discussed this procedure at great length. if you dont know what you are doing you can damage the pump and you wont know it until you trash your engine. for 50 bucks, i opted for the peace of mind in knowing i had a pump with a perfectly positioned pickup that would NEVER fall off and was dead reliable. I just dont think its worth the gamble!  

what happens to summit's pre-welded pick-up if you're using a deep-sump pan or some other non-stock pan?  
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i did love my chevette (first car) more than my first wife
my 94 4.3L Astrobrick would outrun the 99 5.4L f350 i had to drive for work

if that is the case summit or jegs has a pickup that bolts on to the pump body. they are just as good.

one more thing. while you have the oil pan off check the mains, check the gauge (sometimes after the motor warms up funny thing happen) something simalar happened to me not long ago, last check to see if one of the three oil gally plugs just behind the cam gear had popped out. they should be replaced with hex plugs.  

about the gauge. did you bleed the line going from the block to the gauge.  

Well,it sure as hell wouldnt fall off!! if you know that you are using a non stock pan, ( which would be pretty obvious) then you would order the right pump. I was merely trying to stress the importance of a properly attached pickup.  

Hi People,
   Wally is spot on..IF you are in doubt at all and you want feel where the pick
   up is then drop the oil and probe with a length of wire and feel how close
   the P/U is to the pan floor. In my specials I have created I liked to position
   the P/U about 3/4" off the floor woth the screen facing down.
    And YES!! That standard P/U CANNOT BE Trusted the 2nd time "round if you have
   in any way loosened it for any reason. I prefer to have the P/U brazed to the
   Oil pump with common black welding wire which has worked for me just fine..
     I am beginning to like this forum because I can see that I share some of my
   problems with many others..Are we soldiers of misfortune??..Skyeking....  

Did you find out what was wrong with your engine yet?
It definetly sounds like the oil pickup.
It happened to me in 1978 on the first motor I ever rebuilt, it had the same symptoms,scared me to death as I didn't have the money to rebuild it again fortunatly I was driving easy and didn't do any further damage, since then I always weld them in place.