1999 Toyota Corolla Car overheats

Hi, just had my car serviced, they did a new clutch, master brake cyl, flushed radiator & oil change. Car had been off the road for 3 years or so. When I got it back from mechanic I drove it about 3- 4 hours and then the check engine light came on, later it just totally overheated in the middle of manhattan traffic. The radiator was bone dry so I added almost a gallon of coolant and it seemed fine.

last couple times I took it out it runs fine when moving, even at high speeds on the highway. However, when idling or in traffic the engine temp quickly overheats, even on cool days. I checked for coolant leak and there isn't any pooling under car even when parked for a few days, so I don't think she has a leak. Which leaves the fan or T-stat what do you guys think?

Thanks-

PD