yeah, i have a very similar problem. basically, the climate control does not actively manage the output temp very well at all.
when the exterior temp is within a tolerable range (when the need for interior climate control is minimal), the system seems to work effectively some of the time. but when it is either hot out or cold out, the experience becomes much less predictable.
when you first turn it on and the engine is cold, i expect the output to be cold if not just cooler than the user set temp, say 70. but when the engine is warm and the output temp now ramps up to a face melting 90 (it seems like at times) i have to adjust the user set temp to something like 68. now the heat shuts down immediately and it seems like it just pushes through the incoming vent air (with some minimal engine compartment warming perhaps). so, after about 10-15 minutes of relief from the sweltering heat, now it is cooler in the cab than i would like, so i adjust the temp back to say 70. the temp will remain in some state of flux for about 15 minutes, then bam, the vicious cycle starts all over.
i have found that i need to micromanage the user set temp in increments of 2 to 3 degrees every 15 minutes or so in order to maintain a constant cabin temperature.
please help. there has to be some reason for this. im guessing that the thermostat is flaky or that there is a flaw in the constant feedback loop of the entire climate control apparatus. but up here in NoDak, the temps can swing 30-50 degrees throughout the day.