My Mystique did a similar thing, 40 mph 4th gear even on the level ground. The #2 spark plug got a carbon track from the top along the insulator to the base, shorting out the spark under load. Check if all wires are: secure, correct firing order and the plugs should be Autolite or Motorcraft double plats gapped to .052. The coil pack could be a suspect too. I forget the number...Seems the Bosch plugs should still work though if new.
