I have a 1989 Chevy Caprice w/a 355c.i Small block, a TH350 trans, and 3.73 gears. The engine is a 97 truck block, w/AFR Aluminum heads, comp cams, and and Edelbrock intake, stock type pump.

My issue is that when I stand on the throttle, in any gear, any RPM, the oil pressure will go from about 50 or so psi down, down down, till I take my foot out of it, then it comes back up to where it was before. I have replaced the pump, the gauge, tried more oil, less oil, different oil, a different filter. I don't know what to do! I have heard that pickup clearance might be the problem, but I haven't checked yet. Also, my engine leaks oil somewhere, not sure where, but when it was down about 1.5 quarts, the problem went away. when I added oil, it came back. Everyone I talk to is mystified. It will not do it when free-revving in nuetrual, and didn't do it on the dyno. The only thing that has changed betweent he dyno and going into the car is the fact that the motor is in the car. Help please if you can.  

before you go any farther... check the engine ground wires... to the battery and to the body...

on S10's  they had a loose ground screw on the back of the instrument panel...  the gauges would move around when you turned on the headlights...

you can also check the pickup clearance when the drain plug is out of the motor .. fish through the drain plug with a piece of welding rod... or coat hanger...  something to see how far it is off the bottom of the pan.....  

i take it when you replaced the pump... you put in a new screen????     or cleaned the screen... they do get debris in them...

you are running a crankcase breather on the valve cover ... so the pcv system can function properly... sometimes when only a pcv system is used... the vacuum built up in the system can pull the pan in slightly and block the pickup strainer..     when you get hard on it... the manifold vacuum will drop and the crankcase blowby pressure will blow oil out of places you would never think of....     put you hand over the breather opening.. with the engine running..  pull it off in 10 seconds... it should go pop quietly... as the vacuum is released....  the breather should be free to vent and to let more filtered air in...  

Thanks for your help! I'll have to try that the next time I get a chance to work on it.

When I replaced the pump, I did the pump and a new pickup/screen assembly, and there was no debris in the old one. The gauge is a mechanical gauge, not an electrical one, and I have replaced that as well. I'm also starting to think that windage from the crank could be a main cause of the issue. The motor had a windage tray on it when I got it, but I didn't re-install it when I rebuilt the motor b/c of recieving the wrong stud kit or ordering the wrong one, and didn't figure it would matter too much. I'll try to keep this updated a little bit. Thanks again for your opinion and info!