Thank you all for your feedback to my problem. I have recently changed the coupling on the ponies to Kadee's and as suggested this could have been the problem ,I could not find any Mazak faults and the chassis sits square. The track work is also 20 years old and has never given me any problems apart from two point changes, which are all Peco electrofrog. All other locos perform ok.
I have left the two body retaining screws off, and loco has no problem, so I think I am happy with that as points and gradients are no trouble.
Thank you again for all your replies.
Regards
Terry
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