When on a highway drive lasting more than 1 hour, my heat and A/C system seems to shut down. The blowers won't work and adjusting the temp. has no effect. Must turn off car, let it sit for a while and restart it. The systems then work, at least for a while. My dealer has not been able to find the problem. Diagnostic tests come up clean. This happens in both heating and cooling seasons. My car now in shop for last 7 days and dealer is still stumped. I told them to keep car until they can find, and fix, the problem.