By "temp switch" I assume you mean the thing that screws into the radiator, and tells the relays & fan motor when to come on. There is a discussion at:
Alfa 164 Notes that suggests substituting a Volkswagen Golf switch for the Alfa part. The VW switch is less expensive, more readily available, and switches on at lower temperatures. I found one at my local German import car parts supplier (Mesa Performance in Costa Mesa, Ca.). You have to cut the wires off your Alfa switch, and crimp on connectors that attach to the terminals on the VW switch - pretty simple.
Note that these switches really contain two separate switches: one for low temperature and another for high temperature. The first turns on the fan through that resistor, producing low speed operation. Then if the coolant temperature continues to rise, the second switch turns on the other relay that bypasses the resistor, producing high speed.
Note also that that connector on the fan motor, behind the head shield, is another possible failure point. The plastic the connector is made from tends to melt, producing an intermittant connection.
Dunno where you would find a replacement resistor. Not sure if they are available new. APE might be a source for used ones.
