I had a similar problem with the latest Ae 8/14 from Roco. I also have a Märklin Ae 8/14 and have posted a video of the problems I had with that. Cool that you have lots of Swiss locos. I moved to Switzerland from Australia so I could drive them and now I'm living the dream =) I haven't driven the Ae 8/8 before (currently none in working condition that I'm aware of), but I have driven the BLS Re 4/4 & Ae 6/8, both very nice to drive.

This could be caused by poor soldering joints increasing circuit resistance which could cause the motor with the greatest length of circuitry between it and the decoder to run weakly/slower than the motor closest to the decoder. Check all your solder connections before assuming a motor fault, has the second motor been misaligned? If it has then realignment will reduce extra incurred friction. Mine is EU built, but Roco use an assembly plant in Viet-Nam so there could be a QC issue depending on the quality of staff training between plants. The decoder maybe at issue, as Roco do state that one capable of supplying both high surge current plus a higher sustained running current is necessary ie a surge of 1.8 amps and sustained current of 1.2 amps.

I orderd some spare parts for my Iore roco ore train and i got my last part in december a period of 3 years to get all spare parts. i had same problem on my ore train and it was the cable which goes between the locomotive parts.

I’m not seeming to have much luck with ACME stuff. I seem to have little or no luck getting spare parts from them.

I have the same model but it is AC and it is the first edition with number 274. It work nice and I have upgrade the decoder to an ESU V5.0. Regarding the problem, that is probably the motor I have the same issue with a Marklin TRIX Da model and afeter I receive dthe new motor the problem was solved. You can also try to take out the motor and clean the brushes it could be a bad contact there since it works in one way and not for the other. Also review the wires that goes to the electric motor. Are you using this locomotive in analog or digital? if it is digital review the decoder, exchage the one you have for another one and see if the problem persist or changes.

Mabye some BLS coaches will bring your AE 8/8 Muni better in the picture