Random misfire when driving...

[Diamond Hell]

one of my immobiliser's is spliced into the starter circuit

Get them right out of it, especially if they're elderly ones.

The amount of utter crap associated with alarms that I've pulled out of dashboards recently has been amazing.

Have you replaced that earth strap off the back of the head? 

If not, do it - they can look fine, but not conduct.

Go back over the earth connections and check the cables and the fixings for tightness - everything you've described stinks to high heaven of a duff earth somewhere.

[Ben Lessani]

Nah, it's not an older immobiliser - its off my current alarm. I have a few just because of how easy this car is to hotwire and steal.

The battery to chassis, chassis to g/box grounds are spot on (just replaced). I cleaned up the coil to head ground, but I can always stick a new cable on in its place for testing.

What I'll do is just make up a quick circuit that plugs into my immob. relay, so if that stops receiving power when the IGN is +ve it will buzz - only a transistor required so should be easy.

I will check all the dash wiring soon anyway (when the weather is better) and check everything inside, need to replace a few dash bulbs and the handbrake bulb anyway.

[Diamond Hell]

Did you check the earths under the dash, above the fuseboard?

[mk2mark]

Hey diamond, would you mind explaining that? I have a similar problem to the OP. I checked my engines earths and there's nothing over 2 ohms which seems fine to me. What you said sounds pretty plausible though!

[DarnPB]

If it happens again, see what your dash lights are doing. Do they come on as if the engine is not running, or all stay out? I would firstly say that you may have a problem with your ignition leads. Are they old? If so, one may be breaking down, especially if it has been raining. Secondly, if you also have a starting problem as well, then the ignition switch or wiring could be starting to go bad, and also cause the power loss if it goes open circuit for a moment or two. Take the cowls off the steering column surround and have a good sniff and see if you can smell any signs of burning. Even if there is no smell, have a good look at the cables and see if one is loose or if a soldered joint has gone dry.