Hi, Adrian!

This loco does not have the soft-drive sinus, it has the older "rough-drive sinus". It should work and such new locos should not suffer from hardened oil syndrome.

Maybe there is a problem with the flexboard that leads to the motor. If there are plugs at the end, then it may help to remove and re-insert them in case contacts got corroded over time.

Is the decoder pushed down the whole way? Was the decoder replaced? I assume that lights and sound are working, aren't they?
Check for mechanical problems, too - there may be something jammed in the cog-wheels that are visible in the powered truck or somewhere inside.

I have the 39801 - and I had several problems with it, but not the problem you describe here.