I've read all the answers and all are excellent places to look. If all the teeth in the controler are good, pot is OK, contactors good etc., I'd look closely at the plugging relay. It locks the SCR system into pulse idle and after a long delay M range ( or A1 ) pulls in.

Hey boss.

Take a look inside the accelerater box they had a bad habbit of stripping teeth off of the fulcrum in there,
Will work for a whilie then quit

The only thing different in fwd and rev. is the position of the contactors. The normally closed tips may not be conducting after truck warmed up. Check for proper contactor operation. Poor spring in Fwd contactor will prevent normally closed tips from conducting.
The box color is not important. If it is a 020 guts or 350 is what counts. Early original boxes were red. If it has a small black slide switch on the mother board it is the newer 350 box and is not interchangable with the 020 box which does't have the switch.
If the rev contactor is not coming in but the fwd is. I would check the Fwd switch in the controller. It may be hanging and the black box will not let both contactors pull in.

Thanks etharp.
all good points , deseves a twice look over.
as for the pot. it a singal directionl pot.
will keep inform

Thanks danny
the only thing is it a 20 r30tt unit, older then the hills. I my self don't know of that u can re-lean like the easy systems.
thanks for replying to me.

did you try a re-learn on the pot ?