1988 Chevy Beretta high oil pressure

The oil pressure gauge on me beretta is past the 80#s. Before when I stated it it would be really low (around 30-40) and when I step on the gas it would max out past the 80# mark. Now when I start the car it is already maxed out. I had it in park and reved up the engine. The gauge would drop to just below 80 and go right back to max. My question is if it's my oil relief valve where is it.