2000 e150 over heating

I a ford e150 in my shop and it came in for over heating when driving with the ac on. we could get to over heat for half the day. Then when i was driving it got hot light came on and started to run like crap. so i replace the t stat thought it might be stick close. after about two days the customer comes back saying its doing it again. what dose any think it might be? I thinking it might be the radiator. thank for any help.

could be the radiator but one question about the t-stat was it made in mexico? if it was swap the t-stat for one made in u.s. i've seen a bunch of the ones made in mexico never open even when you put them in 200 deg water the thing just sits there.so you can imagine what it does when you install it.