Sadly I have to admit defeat!
This morning I had a go with putting a crown on the wheels, it performed a bit better but still its not reliable enough to be used in combat. Bad design is the problem, the wheels and bearing support are attached to the base plate which is 5mm HDPE, the tension on the track is pulling every thing out of alignment. Even with the sides installed it still looks not right.
I sort of lost interest in this design now...
Think I'll start afresh...new tracked design but it must be a simple one.