PLugs will not cause descibed issue.
I think it is your TPS.... or such, I would get the techs to bench test most of the sensors in the car Maf ,O2,TPS and give the connections a clean while theya re there..
Gear box should not casue revs to drop at isdle as it should already be in 1st at stop/
Good Luck buddy
Yep O2 sensor was the first thing that bounced around in my head.