Sometimes when crusing my pressure drops below 20 unless I let off the gas and let the clutch go then it will pop up and be fine for a few more miles. What would cause this ?

Failing oil pump. More specificaly sounds like the oil pressure relief is getting stuck open or the spring is failing. thats just a best guess based upon what you said

your clutch isnt slipping any is it?

*98 eclipse rs
*2005 srt4 (screw the warranty let the modifications begin)