Grid heater issue ?

So in cold or warm weather Dident matter flash up the truck 12 valve cummins.. An it would always idle at 450 500 rpm.... As it got warmed up the idle would sit at 650 700.... So I adjusted the idle screw so when cold it idles at 750 rpm..... But now as it warms up it will sit at 850 to 900 rpm..... I'm use to all my other trucks ideling high when cold and as they warm up the idle Down.... Confused and it's buggin the out of me ...

94 12 valve 60 hp injectors valve springs done #o fuel plate timing 16.5 degrees and a 5" straight pipe ... Fuel pump turned up with a big air filter...