Problem with DCC loco running erratically

[RodTT]

I'm having trouble with the GP9 I motorised and installed a Zimo MX648 sound decoder into. After working fine for several months it's started to behave erratically. First the loco started moving jerkily for a while, but now it won't move at all in one direction, and in the other direction it races off uncontrollably at top speed. I've tried a factory reset (CV8=8) to no avail. The sound functions work fine, as do the lights, although in one direction there is a clicking noise as the light comes on. As far as I can tell none of the wiring has been disturbed, and I can turn the motor manually in both directions so I know nothing is jammed.

My DCC controller is fine as other locos run as normal. But whenever I try to change a CV setting on this loco it shows an error.

So I'm guessing it's either the motor (one of the cheap Chinese ones) or (more likely) the decoder playing up. I haven't yet dismantled it to try it with a different motor. Has anyone come across this type of thing before?

[ConducTTor]

If it's direction dependent I don't think it's decoder. I would look at motor or gears or something else. If you manually rotate the motor, do you feel a difference between the two directions?

[RodTT]

No I don't, but my next step is to unsolder the thing and test with a different motor, also test this motor on DC.

[ConducTTor]

[RodTT]

The motor works fine on DC. But the decoder shows the same problems when hooked up to another motor. So back to the supplier.
Dismantling the innards has given me another 'opportunity' - I'll try and fit the speaker into the fuel tank rather than inside the body shell for better sound.

[ConducTTor]

[RodTT]

All up and running again, nice and smooth. The supplier gave me a replacement decoder no questions asked.
I fitted the speaker into the fuel tank this time but with no detectable difference to the sound. But it's acceptable.
I also fitted Kadee 705s instead of the previous MTLs. Very nice, first time I've used them.

[ConducTTor]

The supplier gave me a replacement decoder no questions asked.