N13 oil pressure problem

ive got an 1.8l 89 n13 that has an oil pressure problem, when it gets warm (after driving it for 15-20mins) the oil light comes on, and stays on untill the car is turned off then on again (while still moving) then the light goes off.
i hooked up an oil pressure guage to check it while im driving and its all fine and dandy untill its been driven for 15-20mins then the oil pressure slowly goes to nothing (the i have to do the stop and start trick again)
i read that if this happens it could be the main bearings but its not noisy in the least bit.
ive had the head off recently could maybe some crap have fallen into the oil galleries, and how would i be able to clean it out, if this is even the problem??
can someone please help, please?

It could be a number of probems and nearly all of them relate to the oil pump. If there is stuff that fell into the engine oil sump, if could be blocking the oil pickup that feeds the oil pump, it could be a damaged oil pump or even the oil filter... Was the oil filter changed when the head came off?? Best bet would be drain the oil, take off the engine sump(should need to remove the front section of the exhaust) and see if you are mechanically minded, clean out the oil pickup. If that sounds to hard, get it looked at professionally...

Question have you removed the oil pickup in the sump by any chance, i had the same problem in my exa turbo it had oil pressure but as soon you took it for a spin it would go down and you had to turn the car off, i would check you pickup in the sump becasue if it sucks a little bit of air it will cause this problem, and i replaced my oil pickup and it worked perfectly, was a really anoying problem, well hope this helps