OK, thanks; good description of the problem. If it were me troubleshooting this thing, I'd check these things:
1) Carefully unplug the connector on the blower resistor block. It's in the engine compartment, passenger side on the housing next to the blower motor. Look for corrosion on the terminals inside the connector and on the resistor block spades.
2) Check ground G102, which is on the driver's side fenderwell, midways up high. This supplies the ground to the fan switch. Check that the bolt is tight, and there's no corrosion.
3) Chech the 30A fuse #9, for looseness or corrosion.
4) Check connector C204, listed as in the passenger kick panel area; unfortunately the location drawing isn't labeled in the EVTM. This is the only connector between the fan switch and the blower, and it contains all of the blower wires; the wire colors that it contains are:
Brown/Orange - blower power
Red/Orange - blower speed #1 (lo, ground)
Yellow/Red - blower speed #2 (med lo, ground)
Lt. Green/White - blower speed #3 (med hi, ground)
Orange/Black - blower speed #4 (hi, ground)
Although those 5 wires are shown on the schematic as passing through C204, the EVTM calls C204 as a gray 4-pin, so that must be a typo. If you find this connector check for looseness and corrosion.
5) Finally, check the blower switch itself. Besides the connector being tight and no corrosion, look for signs of overheating. Ford HVAC systems are notoriously power-hungry, and I've seen a lot of heat-failed components over the years.
If nothing comes up with those points, pull the resistor block out and look for a loose connection. The resistor block usually gets failed elements, which takes out fan speeds, but conceivably a loose connection there could be causing your intermintent problems.
Good luck, and let us know what you find.
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