Transmission Override Problem

Hey everyone, i have a problem regarding transmission. I have a Mitsubishi Galant with an engine "6A12 - 6 cylinders, 24 valves, DOHC" and coupled with an automatic transmission which right now is having some sort of problem. During normal driving "supposed to be" the transmission is in overrdide mode ("O/D off" LED at the panel is off) but when i do that "engage the transmission to override mode", it works only as long as the car won't run more than 40 KPH, but when it goes beyond 40 KPH the transmission disengages thus producing no torque to the axles, if you leave the car running and let loose the acceleration pedal, after sometime the transmission engages again. Repeating the process "turning O/D on" after it engages will produce the same consequence (at 40 KPH it disengages again). But when the transmission is in override mode (O/D off LED at the panel is on) the transmission works well. Could somebody please help me!!!!!!!!!!!!!!!!
Thanks in advance.....

when trans is in od/on it shifts to overdrive /// if it freewheels after the
shift occurs more than likely the od clutch is damaged.

when trans is in od/ off trans stays in 3rd gear // not requiring od
clutch to engage causing unit to freewheel

you may need some $$$$ work done