95 grand am oil pressure gauge

when first started my car runs fine because the oil pressure gauge is past the half way mark, after driving a while the oil pressure gauge decends to about half, sometimes even lower. how can i get more pressure into the system? and could this be the reason why my car has a rough idle and shakes badly when under a load, inpaticular 50-65 mph

my car is a 95 grand am gt with the 3.1 v6

thanks for any help with this issue