When you flushed, did you get a bunch of crap? If so, you might want to do a more agressive flush with a citric acid solution. Also, a water pump with the wrong impeller is a possibility. Also, how's your fan? A long climb up a mountain with the A/C on puts a big heat stress on, and if you are not moving that fast you don't have very good cooling if the fan clutch is not right.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.