Cam swap cause Oil pressure change?

Sanity check here. Recently swapped from a HFT to SFT cam, the engine(340) had been sitting for a while but was running good when pulled. Swapped to a solid cam, had the heads gone over, valve job new spring and retainers. Cam break-in on the run stand went well, oil pressure right around 80psi @2500/3000 RPM's.
Engine now in the car developed an oil leak at the filter plate. Upon start-up pressure is @ 75/80 on the mech. gage at 1200rpms, zing it a few times and the pressure jumps to 100, then returns to 80. I'm guessing a stuck relief valv. Am I missing anything, hate to have to pull the pan now

Gary