heat can cause the ICV to mess up, could be a couple of other things though.
like the PCV system (oil thins out and starts getting into the intake through leaky valve) or the o-rings along the intake manifold area: engine warms up, plastic expands, orings dont, leak occurs.
if you havent already, replace the distributor cap and rotor, they are cheap and the car runs quite a bit nicer with fresh ones.
Alusil, Dinan DME | ITG Air Filter | Eibach/Bilstein HD | 26/20 Swaybars | Iridium plugs | Depo/ProLumen HID | Optima batt. | no AC | Stoptech brake lines, Frozen rotors, brass bushings, Superblue