1984 Chevy Blazer New engine developed low oil pressure

I replaced my engine with a crate small block 350 about 5 months ago. 194 heads, flat top pistons, rv cam, dual chain timing. It ran fine up until about 3 weeks ago when the oil pressure started fluctuating. It dropped way low at rpms lower than 2000 after engine gets to normal operating temperature. I had used the old oil pump on the new engine and figured that was it. I replaced the oil pump, used the old pickup screen and tube but noticed some cork and silicone debris in the screen area. Now it's better but the same problem except it's below 1400 rpm.
I have 2 sending units. One behind the distrbutor which I replaced at engine swap, and one above oil filter that came from old engine.
I HAVE had a small oil leak I can't find. I'm thinking the front or rear of the intake as the rubber gaskets were squeezing out.
There was no o-ring to replace on the new oil pump (should there have been)?
I haven't heard any ticking noises indicating bearings, valves etc although my exhaust is a bit loud.
I bought a new guage so I can read exactly what the pressure is instead of relying on a 26 year old stock guage and I'm replacing the one used sender. I'm thinking maybe an o-ring between the oil pump and block got misplaced as well?
...any other ideas?
[img:8fd799c01d]http://www.2carpros.com/forum/automotive_pictures/571626_IMAG0057_1.jpg[/img:8fd799c01d]