1990 Chevy Monza Overheats with A/C on.

Engine Mechanical problem
1990 Chevy Monza 6 cyl Two Wheel Drive Automatic 200k miles

Hi,
I have a 1990 Monza Classic 2.0 with 200 thousand km.
It tends to over heat when in slow moving traffic with the A/C on and also when going up a long hill. Once back on level ground, going down hill or moving steadily in traffic it returns to normal. The temp is Ok when A/C is off.
Could it be I need a new or different rad?
P.S. I am in South America where the average temp is 35C daily.
Any help or ideas is greatly ppreciated.
Emmett Osullivan