Oil pressure trouble?

[mike p.]

I have had my Shell Valley for about a year and half.
It has a rebuilt 1973 429 that has been pretty much trouble free for 4800 miles.
In the last week I have noticed a great fluxuation in oil pressure. It can start out cold with 70 lbs showing on the guage, and after 20-30 minutes of driving it can go down as far as 0.
The pressure will come back up to 20-30 lbs after a short time, but will never get as high as it was earlier.
I never hear any rattles or knocks, and the engine never gets hotter than usual.
This problem seems to have started after I put a remote oil filter on earlier this summer?
I'm using 10w40 oil and a NAPA filter, and there are no oil leaks anywhere.
I tried a different oil guage today with the same results.
I'm thinking a bad or going bad oil pump???
Anyone have any suggestions or advise?
thanks in advance, Mike P.