Transmission or what?

My 00' Subaru Forester, an automatic, seems to have started slipping out of gear when I accelerate through a sharp corner, followed by a jerk back into gear after briefly over-reving once the car is driving smoothly on the strait again. The car has 110,000 miles and recently had the cat. converter replaced and had a head gasket replaced quite a while ago. The engine light is not on, nor are any other warning lights. There may be some slight whining/whirring noise going on at times also.
Why does this always happen on a Saturday?.....