I am dealing with this problem in my car. Did a LOT of searches, learned a lot, spent some $$$ and still no positive outcome.
I will make a few questions that may help me to advance.
Problem: Intermitent (almost rare) hot + cold air blown by the climate control system. (hot in the side vents and cold in the center ones) User command: COLD (so the heat is undesirable)
More frequent in traffic jams. (can't understand why)
Usually setting the termostat to a MUCH lower temp fixes the problem.

Actions so far:
At first I simply blocked the heater hose. (don't need heat im my city). Result: No more heat (of course) but the thermostat would NEVER turn the compressor off... car became a freezer.
then...
I decided to replace the duovalve as a relatively low cost alternative. Everything was pointing towards an intermitent duovalve. Result: NONE!

So, I am back to diagnostics...

Questions:

1 By design our climate control systems truly mixes hot + cold air under specific circunstances ? Which ? In other words: is it conceivable to have the compressor providing cold and the heater providing heat SIMULTANEOUSLY ?
2 Why the thermostat (sensor?) would ignore a frozen interior and not disengage the compressor?
3 If the answer to 1 is positive, which are the sensors that command cold production and which the ones that trigger heat production? Could changing them be a cheap solution or is this more sophisticaded? (made thru an integrated circuit, for instance)
4. A bit redundant: Is the control unit "intelligent" or just a switching box ?
5. Any possibility that a failing auxiliary water pump may be part of this mess?

After throwing out the 120.00 for the duovalve I am trying to be more sure with new expenses.
TIA !
Jorge