Cooling fans keep burning relay. Any help?!

I have a '98 with 275k and it started overheating only when air conditioning was turned on in stop and go traffic. As long as I was moving temp would drop but air never did get cold. I was able to limp to my mechanic who replaced fan motors (because they 'weren't spinning freely') and burned out relay. That worked for a few days then relay blew again. They said they couldn't figure out why (fans were spinning freely) and just put in a 'stronger' relay. It seems to be working for now, any ideas on what may be going on?

thanks in advance for any suggestions!