Drove the van up Mt. Lemmon as a test run, and the van did pretty well

It take about an hour from the driveway to the top of the Mountain

Starting at about 2,500 ft of altitude and the top part (accessable by road) is 8,000+

The van can only get to about 35 mph going up hill

Once at the top, it had a few hesitations and it stalled at a stop sign, but no other issues

What do you think could be causing it to stall?

Brakes worked fine on the way down