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. The fuse for the blower is in a little black box in front of the main fuse box. Pop it open and eyeball that fuse. It is a strip element and can corrode. Should have 12 volts on both sides of the fuse.

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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.