Not enough info to pinpoint the problem. Could be related to the idle air stabilizer. Little chrome lookin cylinder thing on the driver side of the motor near the top and forward of the distributor. One electrical connection, two medium size hoses connected to it, attached to the intake with bracket and rubber "ring" holder.
Had a similar problem once and pulled it off and sprayed it out with carb cleaner. Dry and reassemble, and haven't had the problem since.
Or it could be something completely different
Good luck.