A handful of times over the last couple of months my car has apparently slipped out of gear. I'll be cruising along (it's happened in gears 2-5) and all of a sudden the car is slowing down. The engine is still running and the RPM go up if I push on the gas. Normally I'll shift into a lower gear and then with the clutch still depressed I'll shift back to the gear is was in. I took it in and my mechanic said that my clutch is fine, and not to worry about it. There's something unnerving though about suddenly slowing down on the freeway. Any suggestions??