Bosch system e-bike transmission slip

[fenmanctc]

I have a 2017 Cube Step-thru fitted with a Bosch ActiveLine mid motor, which has done approx 4000 miles since new. I have had a number of issues, all related to Bosch components. First, the battery went totally dead. the cells are fine, but the Battery Management System (a pcb) has failed, resulting in a £550 replacement.
The latest problem is skipping gears, mainly in 7th and 8th, under load.. the hub is Shimano Nexus, but I suspect the problem is the front chainwheel on the Bosch motor. This has only 18 teeth and seems somewhat worn. Is this likely to be the cause, or the hub? The rear sprocket doesn't seem worn at all. I am trying to source a replacement chainwheel, which is proving difficult, as it is dished, and the cheapo replacements aren't, so Bosch must have changed the design of this motor around 20016/17. Anyone know?
Can anybody offer advice on my problems, or share their experience of Bosch stuff? I'm a lifelong cyclist, but the cost of ebike ownership is proving crippling.

[Jdsk]

Can you add photos of the relevant teeth?

What type is the chain, how old is it, and have you checked for wear?

Jonathan

[fenmanctc]

The chain is 3 years old, is a KMC Z610, doesn’t seem to be worn very much( have used a chain wear checker on it). I attach a photo of the chainring. By the way, the hub was adjusted so that the yellow dots lined up exactly in 4 th gear.

[bob simpson]

I would say that the cog looks very worn. I would replace it and also the chain.
If the cog is offset you can get cheaper FSA ones.