[98er]

Due to the added load from the compressor in AC and Defrost modes, the engine boosts the IAC signal for higher idle so at least the valve's responding. Being that it is key in idle control and considering the miles on your car, I'd spend for a new IAC and TPS. They are not high dollar, can be installed in a few minutes without need for adjustment, and will most likely fix the problem. There are some enthusiast lists, like SHO forums, that just suggest replacing all engine electronic sensors at 100K to get rid of these intermittent failures.

Make sure after the repair that you disconnect the battery to discharge the system and allow the car to relearn its setttings.
Marc/Atlanta
'98 Windstar
'97 SHO