I would suggest having the whole truck checked over. First, pull your codes (they'll appear on the odometer display if you go into DIAGNOSTICS mode). Usually problems like this are indicated by code ECU 128 which is a general fault code for coolant sensor, injector, EGR or temperature sensor issues.
I would take it to a DETROIT shop and have them hook a laptop up to it so they can find where the code is coming from. If they can't find the cause after a try or two, throw the truck off a cliff and buy a pre-emission model.
This is a known issue. The coolant sensors on those Columbias were known to malfunction, though they didn't always shut down the engine. With the fan kicking on at a low temperature, it is possible that a Barometric Pressure (MAP) sensor could also be causing the issue. I would suspect the latter, especially since it's happening when you gain or lose altitude via a hill climb or descent. There are two of them in the air conditioning circuit and there is a third one on the intake manifold itself. CAT engines have a similar set up.
If you run the air conditioner a lot, have the Freon level checked and make sure your air conditioning system is not leaking when you take it in for service.
Checking the codes will certainly help, but often, sensors would only give the code ECU 128 (which is a general fault code) on that model of Columbia, making the issue very hard to pinpoint.