2001.5 Passat Wagon

My 2001.5 Passat Wagon idles weird at a stop. When I am at a stop light, in Drive, it jerks like it wants to go forward. The idle goes up and down and feels like it may stall. The really weird thing is that when you turn the air on high, it stops? And sometimes when you turn the air OFF it stops. Does anyone have any ideas?!