Yesterday on the highway my manual transmission 2000 Focus ZX3 suddenly lurched and lost power. The engine was still running and the RMP's stayed around 3000 but the wheels weren't getting any power. Shifting caused the car to lurch but it wouldn't engage again. I turned the car off and re-started and it ran fine for awhile and then did the whole thing all over again. I got home but repeating this whole process about a half a dozen times. It seemed to "disengage" only in fifth gear and usually when the slope of the road changed, but I'm not sure if that was actually the case. Does anybody have any idea what the problem could be?