Did it again this morning. Two more clues - 1) the ASR light / limp home mode first kicks in when I hit the brake, and 2) if I don't reset the fault code it stays in the ASR light / limp home mode, but reset the code and it goes back to normal operation. Seems to me unlikely that it's the engine actuator - why would it happen when I hit the brake? I would not mind at all permanently disabling the ASR if it would get rid of the problem. It seems such a nuisance on an older car.