94 gt auto tranny shifting problem

This car has me puzzled? I can be driving along on the interstate on level surface and it will shift from overdrive back to drive and stay in drive for 5 to 10 miles and shift back to od. At the same time i noticed the indicator light on the console will go out also. I changed the fluid and filter but still the same, anyone ever had this problem before?

slight possible, vacuum modulator is going out, or you have a leak in the VM hose. Other possible is TCC [torque converter control ] is messing up. both controls mount to tranny, the VM is hosed, the TCC is electric-plugged.