oil pressure drop once warmed up

[Rattletrap]

after about 20-30min of driving, aka when the oil finally gets heat in it. I have oil pressure drop.
starts out cold at 40ish and builds like it should to 60+ and as its driven the OP gets down to 20 and will build to 30 but you can start hearing the top end rattle.

I put a "new" pump on it but who knows.
I do not think it would be the o-ring since it does have pressure till it gets warmed up.

Thought was to replace pump and if that doesnt help then I guess i will have to rebuild, URGH!