Just bought car a month ago. Had timing belt changed and 3 days later it started overheating.

I drove car for two weeks to school and back (40 miles one way)check engine light came on. Took to autozone and it read evap leak(small). They suggested replacing gas cap, so i did. Light was still on, so I took to mechanic and he found a small tear in hose, and replaced it. I had my timing belt changed while there because internet said to replace at 105000 miles and I wanted to make sure. I drove to school, and on 3rd day it started overheating. My mechanic can't figure out what is wrong with car. Replaced thermostat twice, radiator cap, flushed radiator, new antifreeze. It doesn't overheat when idoling, but after you drive it for a while it shoots heat gauge all the way up. Turn heater on and it immediately goes down, at least half way. They said that the way it acted and overheated makes no sense to them. It has been in shop for 3 weeks now, and they still can't find the problem.