I've re-adjusted the idle on the back of the pump 3 times over the past month or so - the problem is it slowly creeps back down. I set it at 800RPM in drive with the A/C on & after a few weeks it gradually goes back down to around 500RPM for some reason.

I've snugged the locknut tight enough I think and put on a new fuel filter but problem is the same - haven't cleaned out the pre-filter/heater yet, that may be next. Is there another adjustment on the pump that controls idle speed that could be slipping or something. Thanks for any help.