jstck, I have absolutely no idea. Tacho Output was turned off, so I don't know it was even getting a reading in the first place. It was strange as if I slowly increased the revs it read true, if I booted it it just shot down to ~600 until I lifted off completely or slowed down the rate of acceleration.

4strings, will do, but I'd like to have it idle on it's own and be able to turn it on, without the need for me to keep modulating the throttle. I'm guessing Open Loop (Warmup) will achieve this.

Tuning fuel by hand in the idle area is generally recommended and the first place to start. Open loop after will help yes.

I had a quick play and managed to get it hitting the AFR targets at idle and was very smooth, think I'll need to look at acc. Enrichments later on mind.
Going try again with the idle control tomorrow and see where that gets me.
Thanks

Just a guess, but it could have been that the tacho signal wire was picking up noise from some other cables that move in sync with engine rpm (injectors or ignition), and that the noise picked up changes enough or gets drowned out when you step on it that the tachometer loses it. If the Megasquirt output is high impedance when disabled, that cable going to the tachometer would make a decent antenna.

As with lots of electronic components in cars, probably the tachometer just does it's best to get a reasonable rpm signal out of whatever it gets, since there will always be some amount of noise in it (because car).