I just bought a 2003 Legacy wagon. When I test drove it, is was evening so I didn't run the air. I turned it on when sitting in the lot to make sure it worked, and it does fine. The problem is that when the air is on, if I am travelling between 35 - 50 mph and let off the gas, it jerks when I reapply pressure on the pedal. It's not just a subtle jerk, it's fairly significant. Enough to bug me anyway. It does not do it with the air off. My theory is that when the air is on, it takes away enough power that it wants to jump out of overdrive when I reapply the gas. I may be way off base, but that's the theory I have developed. Anyway, if that's it, how do I stop it. If not, then what is it?
My 2001 does the same thing, but only in gears 1-3. when coasting in those gears, the car jerks significantly when I put my foot back on the gas. I'm sure the same happens in 4 and 5 but there is so much momentum it doesn't do anything. My theory is something about transitioning from being slowed by the engine to being accelerated by the engine is not going smoothly in the transmission. Could it be clutch slippage? I hope not. Sorry I don't have an answer for you just sharing my experience with the same thing.