Oil pressure

I have a 92 silverado the oil pressure gauge will
read 25 to 30 psi when cold after warm when I stop
I will read so low the check gauge light comes on
once I rev it or take off it goes up to 20 psi or so I
was told it might be the cluster my temp gauge will
sometimes read over half way if I tap on the front
of the gauge the temp needle will go down it has
93000 miles I knocks a little upon cold start
nothing after warm does idle a little rough when
stoped and pressure reads low been driving it like
this for over a year this leads me to belive its not a
pressure problem but unsure