WOW! Everyone's hopping on this bandwagon!
You may get more responses with your own thread. To me I would suspect the blower motor itself is bad, it should be tested (see if there is power to the motor in the failed state.
Rafi: Usually with a large fluctuation of the temp gauge, I suspect the thermostat. Possibly the thermostat that was installed is defective. What is the gauge fluctuating between (what are the high/low readings after driving the car for 10 minutes?)
Brian: You can have either the high pressure or low pressure switch shutting off the AC system, either because the pressures actually are high or low, or either the high presure switch or low pressure switch is bad. There is also what is called the "Klima" relay which may be bad, this is what controls the clutch. There could be a problem with the rpm sensor in the ac clutch also.
I'm not sure which engine sensor you are talking about, can you post a picture of it? Another possiblity is the "push button panel", this is the part in the dash which runs the ac, it's also the part with the buttons on it to activate the system. On the fan not working, you need to test is as I outlined for Marty; get it in a failed state and see if there is power going to the fan, if there is, then the fan is bad.
I'm not sure what computer chip you replaced in the 300D, usually we speak in terms of a component being replaced, not an individual chip, although some poster here do get right down to saving money by going into modules and replacing and resoldering things, more power to 'em if they have the time to invest in diagnosing and repairing circuit boards, it goes beyond what I like to do.
In regards to "heater is on but no hot air", what I usually think of is what's called the mono-watervalve. There is a rubber diaphragm in the mono-watervalve that can rip, which can cause either too much heat or no heat, basically the temp in the heater is no longer regulated at all. The part that rips in the water valve is replaceable as a seperate part, I can tell you how to inspect the water valve if you like, or use the search feature, I've explained it several times already here.