compressor could be going bad, if the compressor is starting to lock up it can strain at idle cause the rpms are not high enough to keep it going. when your driving the rpms are high so there is more power to turn it. i just took mine out, i bought a remanfactured one (learned my lesson dont do it...) and it worked good enough to blow cool air (cooler than the 100-120 degree air that is in my car when i get it), but if i stopped or was in neutral it stalled my car out. even if i was in 3rd, if i held the gas at the right spot the compressor would cause my car to slow down then it would speed back up then slow down and back up again