I'd bet on the regulator starting to fail. I had this happen to me. The only problem with all of this is the regulator is buried under the fan in the cowling box. Pain in the "A" to take apart and get to, especially in an older car (plastic parts can be really brittle). Also, just to be sure, check the connector where it comes thru the firewall(behind and to the right of the brake vacuum boost can). If this is corroded, it can give intermittent control voltages to the regulator and it can cause these symptoms. Make sure it's clean. Check the voltages here as well. The red wire connector should be system voltage, the yellow (with the fan on high) will be some where near 7.0-7.5 volts. The regulator is the ground source for the fan. Good luck. I wouldn't bet on the ACC failing just yet. BTW, make sure your voltage regulator is regulating the system voltage properly. If it's too high, this will also cause the fan regulator to act up.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.