My '99 refused to idle correctly after it got cold out, it would stay at 1000 rpm until put in gear and drop afterward but still pull higher than normal. As odd as it sounds, after I replaced the thermostat, it went away and now idles at 550, even at -20. I believe it had a 160 F thermostat in it that just went bad. It had done that for two winters, no matter engine temp. Just my tidbit

 

i have a feeling this is the case with mine. as soon as temperature outside drops RPM's go up. and I do need the tstat replaced. hopefully get that done later today.

will update you guys.

 

Mine didn't have the right thermostat in it, it would sit solidly on the 2nd big mark instead of nosed up against 210 like it should be until it started to die, then it was it between the 100 and 2nd mark.