I have a newly rebuilt SBC 383 that is having some weird oil pressure issues. when its cold it has lots of oil pressure but when its warm at idle it goes down to 20 psi when in gear at the light and 30 if I put it in neutral, but that not the weird part.

The weird part is when I'm driving down the highway. my oil pressure gets worse above 3000 rpm. at around 3000 rpm it will cruise around 38 psi, but go any higher and it starts to drop. at 3700 rpm its down to almost 20 psi. I noticed this the first time I had it on the highway after the rebuild. The oil pump was not changed during the rebuild

Engine bored out 0.010, with new rings, pistons, and all new bearings done professionally after a piston failure. IT has a high capacity oil pan so i don't think its starving for oil at higher RPM. The oil pressure line run right from above the oil filter to the gauge in the truck. IT is the plastic line that came with the gauges and not a metal one.

Not sure where to start looking for the problem. I'm thinking I'm just going to end up putting a new oil pump in it and maybe switching that line to metal. Anyone else have any ideas of what could be causing my issue?