The fuel pressure on my M30 will typically sit at 42 lbs. Just right.

Occasionally after the car has been running a while, the FP will drop to about 35 or 36 lbs and stay there. At least until the car is off and sits for a while.

The lower FP messes up the mixture and the car runs a bit leaner than I would like, especially on idle.

Any ideas short of replacing the fuel pump? The FPR has been swapped with a different unit and the behavior is consistent so I don't think it's the FPR.

Ideas?

Jeff