A software failure (mentioned above by Viper X) of the fan speed only is extremely unlikely.
However, since this fan speed signal travels from the electronic module to the hydraulic modulator(right name?) via a dedicated wire, a connection problem could cause fan speed problems. Also, a bad hydraulic modulator (hope I got the name right; it's the gadget that allows hydraulic pressure to the fan motor based on the electrical signal from the electronic module) could also cause overheating. I don't know if the modulator has to have a ground return path for the signal, or if the signal return path has a dedicated wire.
If you want to measure this signal and/or check your fan speed, simply turning on the A/C with the engine fully warmed up will probably turn on/up the hydraulic fan motor signal.
The signal is a pulse width modulated signal, and you can measure it with an ordinary meter. The higher the signal voltage, the faster the hydraulic motor should run. If I'm remembering the electrical manual correctly. The signal probably should be lower than 12~13 volts, as hydraulic motors are VERY powerful.
Anyway, I'd measure this signal while observing fan operation before replacing parts. A dealer or someone with a DRB III should be able to put the cooling fan into a test mode where the signal can be measured and the fan rpm can be measured and compared to known good values.
One other thing...because the fan runs on power steering fluid, any kind of air or lack of fluid in the power steering system will cause problems. As will slippage of the power steering pulley, either on the serpentine belt or on the pump shaft. So check your belt tension and condition, as well as pulley to shaft tightness and pulley key integrity. Assuming it has a key; most power steering pulleys do, and keys have been known to break, fall out, or not get reinstalled.