idle air control valve problems on a 91 5.0

[brett p.]

I have a 91 5.0 mustang with a 5 speed. The motor is pretty much
stock execpt for roller rockers, headers, pulleys, the mass air
meter, 65mm throttle body, and a cold air intake. The vehicle was running fine with this combination. In fact it was running quite well until I put a Trick Flow street intake on it about a month ago. What happens is that the vehicle idles good and runs very well, but when I start to come to a stop and lets say I'm running 2k rpm and I push the clutch in and take my foot off of the accelator it sometimes wants to shut off. The vehicle cranks up fine and runs normal, but its annoying to have it shutting off while I'm coming to a stop. One mechanic told me it was either a vacume leak or the IAC valve. I checked for vacume leaks and could not find any. I replaced the IAC valve with one from Advance Auto Parts, and it doesen't shut off as much as the stock unit, but every once in a while it will. Any help will be appreciated. I have to fix this problem. Thank You.