lost oil pressure

[sledge33]

yesterday we started my engine for the first time.i bought the engine about three years ago from a machine shop and it had been rebuilt.the engine sat in my garage until i installed it in the car before it went to paint and i never got it running.the car was at paint hell for over a year.now that i got the car back we were going to get it running yesterday.everything was going good,we primed the pump with a drill and got 60lbs of pressure, and so we started the car and the oil pressure was around 70 lbs.we let it run for 15 min at about 1500 rpm. when we went to idle down the oil pressure went down with it,all the way to 0. we shut off the engine and checked temp because we didnt think the temp guage was working. the engine was still cool to the touch.we would start it back up and oil pressure would go to 20-25lbs and slowly drop over 10 -15 seconds, i dont know if it would drop to 0 as i would shut the car down when oil pressure would drop.
any ideas?
since it sat so long could it had some rust to clog up filters?
the engine oil was clean when i drained it.
i was told the plugs behind timing chain may have came out.
any help is appreciated.