So when I am maintaining a steady speed for a while and I shift out of gear into neutral the engine rpm sometimes drops below idle speed. Idle is around 900 for my truck but sometimes the rpm drops between 200-500 and sometimes shuts off all together. It tends to happen more often when the a/c is turned on. Very rarely does it happen with it off. The truck is a 2001 Ford Ranger XLT 2WD 5 spd. manual. Any ideas?