I have a 2002 Freelander w/ 2.5L KV6 petrol engine. After driving 2 hrs at 75 mph from San Francisco toward Lake Tahoe, climbing from 5000 ft to 7000 ft the engine begins to lose power. Temp guage doesn't indicate overheating...no knocking or other noises...an occasional miss but basically just lack of respons from foot to the floor. If I stop and let it rest for 15-30 min then I can proceed w/o problems. What's happening and how do I correct it???

You would be betterer asking on the main Freelander forum over ere: Land Rover Freelander - Land Rover Zone

Use an OBD2 scanner on live data setting. You want to monitor the intake temp. If it's climbing above 30°C The power will drop off some what. That coupled with the drop in air pressure would make a difference. As to why it corrects itself after an off period is a mystery.
If you are getting an occasional drop out, it's worth getting that looked at. It should run without any form of misfire / drop out.