heater goes cold. frozen coolant?

[PDESORM]

Hi everyone,
This is in regard to my girlfriends 2002 Montana, 3.4
I've read all the posts on heaters going cold but they don't seem to fit this problem.
When temps outside are -20 to -40, like today, the van starts well and runs at normal temps. and gives heat for 5 to 10 minutes. Then, while driving, heater goes cold and engine temp goes up drastically. I'm watching on a code reader. Top pipe into rad is warm, bottom one is cold. I topped up the rad last week with Dex. II mix. There appear to be no leaks, there is fluid is in reservoir when warm. I'm wondering if the mix may be too diluted and freezes while driving. There are no bubbles or air pockets in the lines. No oil in DexII and no DexII in oil. No odd smells, and exhaust appears normal. everything works fine in warmer temps.
Any ideas???
thanks!

[sisson_d]

Sounds like a bad thermostat.

[PDESORM]

I replaced the thermostat (outside in a sleet storm) and the water pump. Flushed the rad and engine, bled the system, it worked for 20 minutes max and then started the overheating. Rad was full of air pressure and had to be bled again. The whole system was full of air! Final verdict: bad head gasket. Bottom of the van is badly rusted so mechanic condemed it. I, however get to use it for spare parts on my 2004 Montana!
thanks for the help.