Hi guys, I was hoping some of you could offer me a bit of advice on a wee problem I'm having with my sierra. I bought it recently, my first ford and first turbo car so any advice would be appreciated.

Heres the engine spec:
T35/-63 with -34 actuator
3 bar map
AH RS500 type intercooler
Grey injectors
Stage 4 chip with closed loop conversion
Cometic multi layer headgasket
Group A Coil
Uprated plugs
Group A Fuel pump
Roose hoses
Induction kit
Spec r breather system

Car drives and goes perfect until it's driven around town or anywhere at a v.slow speed, it then starts to miss and splutter. As soon as its taken back up to a decent speed though with plenty of air flow it clears itself out and runs fine again. I just replaced the distributor cap and rotary arm as I thought this may be the cause of it but the missfire continues. Any thoughts?

Cheers
Bob