MK1 K-jet missfire.

I dont know if anyone can help me on this but i am well and truly stuck..

I have a '90 Cabi running k-jet. I have had a missfire for the last 12 months that i can't solve..

When the car is cold it runs sweet as a nut, no miss fire at all, as soon as it warms up and I try driving around town or below 2500rpm it,
miss fires like hell. I did the head gasket a couple of months back, (it was miss firing before i did the head gasket) when i had the timing light on timing it back up it appeared to be dropping the spark on no1 cylinder every say 3 revolutions of the fly wheel.
I have tryed changing the dizzy, ignition control unit, coil and the wiring between it all but still have the same miss fire.

Any advice would be most appreciated.

Cheers

Have you checked the mixture when it occurs?

I take it to you have checked the most obvious cause, a rotten fuel filler neck - the Achilles heal of the mk1

have you checked the compression when hot