Dapol class 26 issue

[Wayne 56089]

Hi guys just need to pick your brains 

Decided while on holiday to add to my loco fleet and grabbed a couple of bargains and acquired a dapol 26, 007 in coal sector.

Tested in on dc and worked fine, fitted a chip and managed to get it all back together with lights still working  now I'm having a problem with it on my layout!

I've only got set track points and all my other locos dapol 56 and 58 and farish 101 and 47 all run fine over them but when my 26 runs over them set straight it causes a short circuit! Runs fine when turning it only happens when it's going straight on.

I wondered if the back to back length was a bit out but checked that and checked them against my other locos and there exactly the same!

Any ideas?

( if this is in the wrong place then my apologies and please feel free to move it )