strange oil pressure drop.

I've searched all over for an idea of what causes a fairly large oil pressure drop in my engine. Cold idle is the usual 55-60 psi, hot idle 20 psi. Now the unusual part, at 1800 rpm i'm making 40 psi but put it under a good load (i can lock and unlock the converter to provide a load i can turn on or off) and the same 1800 rpm only provides about 25 psi, if i lift off the throttle the pressure jumps back to 40 psi, hit the gas again (not WOT but just enough to provide a good load and pressure drops again. this is happening with hardly any change in rpm. Under a heavy load like this i don't make it back to 40psi till about 2300 rpm. I've verified this with a mechanical gauge. only about 6k miles on rods mains pump cam and lifters. pump is high volume. new 10w-30 oil and mopar filter. just slightly less pressure drop with 15w-40 but the same 40psi peak. now i know the 10psi/1k rpm rule but factory spec is 37psi minimum @ 1600 rpm. anyone else ever experience a similar situation?