overheating

i have a 2001 Durango that's been overhaeting recently. had the thermostat replaced (it was bad) but it's still overheating. had the local auto shop check it out and they said they couldn't find anything else wrong. (checked fluid levels - no leaks or burn spots, check the fan, the radiator, etc.)

any ideas? it doesn't overheat every time i drive, just when it's really hot out and i run the A/C

We had same exact problem and it was radiator sludge. Took radiator to shop and had them clean it out, fine ever since. apx 30K miles ago