SeanR

Swap in a used cluster and see if that changes things. Much simpler to do that than try and experiment with this one.

John Speake

As well as the tach, the kick down relay (if fitted) also is hanging on the same rpm line that goes to pin 1 LH.

If the GTS is an auto, try removing the KD relay, if a 5 speed make sure there's nothing in the KD socket.

siscogts

I saw a similar issue on a BMW 2001 M5....bad knock sensor on 1-4 cylinders, a loose bolt on knock sensor on the other side....
But here I am just a newbie and maybe LH behave differently about faulty knock sensors....
Francesco