have you looked at the throttle position sensor (i think that's waht it is)? I had an old mystique that would randomly change gears...often banging when it did, or just stay in the wrong gear when it felt like it. There was some sort of sensor that the mechanic changed, that was supposed to sense how much throttle you were applying and select the appropriate gear.

1999 Silver Frost SVT #609 of 2760 Quaife, lightened SVT Flywheel, SPEC stage II clutch, removed resonator, k&n drop in - various other goodies too.