2001 saturn sl2 overheat

my saturn sl2 is in pretty good shape, but the only major problem is the overheating. it overheats when idling or driving locally, but is mostly alright on the highway. i thought i had the problem licked when i was told i had a crack in my radiator, so they replaced it; the problem, however, returned. the only solution to this problem that i have found works is to keep the A/C or the front window defroster on. this leads to me leaving the defrost on low and middle-temp about 90% of the time, and A/C in the summer the other 10%.

any clue what could be causing my problem?