Cosworth V6 problems :(

[cossiemk2]

I've built an 2.9 24v Cossie in my Mk2 Escort and it goes like hell but there's a problem.
When I decellerate for a traffic light an I depress the clutch the engine stalls.
When I start it up again it runs fine for a while but it comes back regularly.

I've cleaned/replaced/checked everything but the problem remains

WHO CAN HELP ME!!!!

[boosted ls1]

Sounds like there's no air getting past the throttle when it's closed, ie idle air by pass pipe. If you did a test with the throttle slightly open would the car stall or run ok?