Thanks Dan. Disconnecting the electrical contact at the water valves reverts to full heat as is should.
All water hoses (upper/lower rad, to/from heater were replaced only a few months ago, along with the water pump, so I think this is unlikely the cause.
The temperature switching always happens when the car idles for a few minutes or when crawling in heavy traffic. Air flow changes as well. When it switches from cold A/C to hot, I get hot air comming out of the center vents, which I don't think is normal.
Could it be related to some vacuum operated flaps under the dash?? Thanks for your help