1989 Chevy Truck oil pressure

It sounds like a bad oil sending unit, to be sure you have oil pressure. You may want to hook up a manual gauge

also remember there are 2 oil presure sending units the one by the distributor is for your oil light and its like 15 dollars at autozone and the second one is down by the oil filter and it gose to the gauge its like 25 bucks at autozone these sending units are not the same .... i made that mistake and my truck was doing the same thing ..... most likely the one down by your oil filter is going bad and evetually it will start leaking like mine did and all together it will stop working but mind you it could blow out your other one so get it changed asap