I just ordered delivery valve seals and I'm gonna rebuild the delivery valves before I go ahead and pull the pump. To me it seems that bad delivery valve seating/seals *could maybe* cause this problem, because injector lines would possibly leak low viscosity fuel back into the IP plungers between injections at low RPMs.. I.E. in the time between injection on #1 and then back to injection on #1 @ idle rpm, thicker fuel would not be able to leak back through bad seal in this amount of time... Also seems to make sense (to me at least) that the injectors require greater throttle position with thin fuel to pop and maintain idle, just an idea.. maybe it works maybe it doesnt, will post when I switch them out... if it doesn't work I have a brand new-sealed set of delivery valves for my replacement pump.