1995 Pontiac Bonneville oil pressure dropping at a stop

Fun and joy. Just bought the car last night. Took for drive and was fine-actually great. The top end of the engine has been completely rebuilt. Runs like a champ. But.
when I got off highway last night and came to stop sign, the oil pressure gauge started dropping in the red and therefore the alarm started chirpping. Rather annoying. But then once moved again went to normal operating range. Also on this, pulled up to the local burger king at a stop and dropped again, but if I put it in neutral it went to normal range, but if in drive and not moving with foot on brake dropped.
please help and advise and hopefully I didn't buy something that will bite me in the end