Is the time before M speed comes in the same each time? 10-20 minutes? Is there a noticeable difference in the SCR speed and the M speed. Does the truck travel a long distance. There is nothing wrong with running in top SCR range. The SCRs work harder and the heat sinks stay hotter. Speed might be a bit slower.

Sounds as if the encoder (drive motor direction and speed), if there is one, would be the issue. Feed back from the encoder and amp draws across the drive motor are used to determine if hi speed is allowed.

I thought Raymond trucks were using transistor drive by '92. Guess not the 020, which were built in Canada.