This is really a stretch, but based on you turning on your fan, this
will cause it to increase idle to compensate for rpm drop from the
additional load added if the AC compressor (if in AC or defrost modes)
turns on? This might stablize the idling.
Anyway, how are your plugs? Wires? Mine used to stumble at
lights/stop signs even with only 18k on a new plugs (truck 96k) I must
have leaned out one of the cly once coz the tip was almost gone on one.
New plugs/wires fixed all. (121k now)
Ted