S40: 2003 S40 Stalls on increasing altitude

[JimOfMex]

I have a 2003 S40 with manual transmission. I can find no references anywhere to my exact problem. The car runs fine when I'm just running around town even on the highspeed highways. This is about 98% of the time. Once or twice a year we drive to the beach - about 3 hours. We live in the mountains of Costa Rica. As we descend to the beach there are no problems. When we climb back home, the car begins to lose power and even stalls at times after about 1 1/2 hours. We have to stop every so often at the side of the road. Then, the car will resume normal operation for another few kilometers before doing the same again. When we finally get back to our home town of Cartago, the car resumes acting normal. We are no long climbing.

The car has been in the shop 3 times for this problem with no resolution. Can this be the barometric pressure sensor, if there is one on this model? Volvo repair prices here are sky high. Can I order one on-line and install it myself or will the computer need to be told about the change?

Thanks in advance.