Thanks guys & thanks to all who replied.

After reading Stu's post, I quicky put a new thermostat put back in. Still missfired badly.

So I took out the plugs. The electrode on number 1 was broken off! I replaced the plugs with old ones I had in the garage - it's much better but still wont rev all the way without missing.

I have the following questions if anyone would be kind enough to offer advice it would be much appreciated...

1. Is the fact that the electrode has broke off an indicating of a bigger problem waiting to happen? Lean mixture? (I have pics if there's a way to post them?)

2. What plugs are best and what mileage should I expect? I'm running AB software on Green injectors. Engine done approx 30K - these spark plugs (Motorcraft AGPR 902P) have only lasted about 4k miles - the other 3 look fine.

3. After the engine was built, it was running on Motorcraft Super AGP701C which never missed a beat in 25K miles - I've been told these are motorsport plugs of some kind and are no longer available. Is there a replacement ?

Thanks in advance,
Boon