Transmission problem

[Mikee4000]

My 1991 Hilux Surf has developed an Automatic Transmission Problem. It started with the lockup torque converter randomly disengaging for no apparent reason, if I turned the over drive off and back on again that would resolve the problem for maybe a minute or a day or two. This has gone on for about 6 months now with no apparent change. Now it is becoming more frequent and  also does not shift down when I come to a stop sometimes, if I shift manually it will shift down most of the time. The last thing to occur which has happened in the last couple of days is the over drive off, warning light flashing for about a minute and then stopping. I have been monitoring my driving closely to see if there is any kind of pattern to this problem and have come up with nothing. whether its cold or warmed up seems to make no difference as well. I would greatly appreciate any input into this dilemma.