I am running the Walbro GSL-392 pump with the FPR mounted on the inner fender as usual and set up like yours use to be using -8 feed and -8 return. I was having issues too where the FP will be high and after the car warms up it would drop a bit. I have been chasing my tail on it like you did. Recently I put a restrictor in the return line and that seemed to help but then again after it was running a while and got hot it returned back to the same issue, not as bad but still there. When I get back out in the garage I will be setting it up just like yours is.
Did your FP start high and drop or did it do the opposite and spike? What exactly was your issue with the FP?
I also heard that Liquid filled gauges can sometimes give a false reading when they get hot? Have you heard this before? Do you run a liquid filled or just a regular? Any input on the subject will be appreciated.