Intermittent fuel regulation

I performed a 255lph fuel pump transplant on my 1990 GT. I have done this operation once before on this same car (with a stock replacement pump) as well as once on my old Aerostar with good results. However this time I have a problem I am hoping someone with be able to help me with. After replacing the pump, my fuel pressure gauge showed slightly more fuel pressure (from 40 to 43 psi), so I adjusted the Kirban regulator. At first the fuel pressure dropped as expected, then the gauge needle jumped and began fluctuating between 44 and 50 psi with no adjustability. This condition caused a high idle (1000 vs 750 rpm). I checked the fuel return line for kinks and found none. The next day, the regulator worked perfectly and I was able to set the fuel pressure. I took my car out for a spin and when I returned, the fuel pressure again was erratic and the idle higher than normal. That is my problem:banghead:. I hope someone can throw me a bone. Thanks in advance.

Maybe your fpr is going out.