hey iceman,
your problem is most likely the full vacuum location of your vacuum line. since it see's full vacuum all the time and you have a "possibly" a big cam in it when it warms up or when you are revving it for the break in time the modulator fails due to too much vacuum on the valve. move it to a timed vacuum port on the carb to see if it clears up. or see if it is ajustable and adjust the valve accordingly i beleive they are adjustable. hope this helps
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hmmmm the sound of wrenches turning and a air impact gun going... slowly out from the garage emerges a luxury muscle car that is anything but... it is.. the monte carlo with a new edelbrock performer rpm power package and new 350 block stroked.