Mumford linkage.

[DaveEFI]

Happily the engine is running sweet now after years of grief. The problem would only happen on track and not on rolling road! out of 6 different tuners only one diagnosed the problem to be faulty wiring done by the original installers of the EFI system It has cost a fortune to sort out. Very annoying when you pay good money to so called experts who mess up on the fundamentals. Thanks anyway Mark

Doesn't much surprise me, given the standard of some of the 'pro' wiring I've seen. Bit of a hobby horse here.
I'm not familiar with your ECUs, but MegaSquirt have a logging facility including a tooth log. Which is an easy way of seeing any problems with the tach signal(s)

[mgbloke]

Thought I would update. For various reasons I've not used the MG since 2019. While its been off the road I have fitted a dry sump system from John Eales and done further front suspension modifications including dropped spindles and stiffer springs. First time out a few weeks ago and it just makes me laugh how well this thing goes round corners.