My english is bad!!

Well, I have 524 td (automatic, full) '93. My problem is overheating but not allways, In the city driving everything is fine, on highway road also is OK, but when I start to climb to the mountain, my temperature start to climb too!
I have change " VISKO" and the "thermostat" and one guit has been closed and i fix it, but I didn't determine problem.


Please help!!!