One last idea, maybe it's point bounce (the coil wont care), does it calm down after 3500rpm or continue? I don't know how to fix that. I don't use points on mine.

Only other thing I can think of you probably can't do much about, the coil rings (electrically) when the points open, maybe the input filter on the tach isn't quite right.

That's about the limit of my knowledge I reckon.

Thanks for the help and ideas guys,

I went back to autobarn at doncaster tonight and i was quite surprised at the service i received. First they got out the demo model of the tacho so i could rule out whether or not it was the car or tacho that was faulty and it turned out to be a car issue as it was erratic again. Then they ran the issue past several of the staff until one of them knew about it, he then spent 20min checking for tacho filters that he thought may work but was not 100% so he went and checked emails about the same issue with a previous customer. After all that failed that grab a coupe of different tachos and we went out to the car and started trying them out, by doing this we found that the SAAS brand and another does not work well with the series 2 bluebird dizzy, they have experienced this before. After all this the conclusion i came to is that i would have to spend the money and get an autometer tacho, expecting to be told i had to keep the SAAS tacho as technically there is nothing wrong with it and its a compatibility issue(as i have experienced elsewhere), they quickly informed me that i should remove the tacho and return it for a store credit against the autometer.

So they are few and far between these days but there are people who know how to provide service.

Will try and get more info on why it interferes and let you know.

Guess I'd better pay my sparky bill happy sparky happy wife happy datto blah blah blah