Mitsubishi Repair: 1991 Galant SOHC Automatic, shift linkage, transmission shops

Question
Car will downshift suddenly while driving (could be 40 miles on highway at 70mph or could be in town just pulling away from stop light) into a lower gear and stay in that gear alone regardless of manually moving shift from D to 1 and back. Have just replaced trans fulid and filter, fluid looked realatively clean. Car has 89,000 miles on it total. Any ideas of things I can check before taking to service shop ?

Answer
Tony,
Unfortuntely when it comes to transmissions (especially auto trannies), there is not much the average person can do other than what you already did.  But you should call around to different transmission shops in your area to find out if any offer free inspections or diagnosis.  Your problem may be as simple as a loose shift linkage, or it may be as severe as the need for a transmission rebuild.  Your mechanic should also check your car's main computer and the transmission computer.  It's possible your problem is eletrical.
Good luck!