1995 Pontiac Grand Am overheats when driving slow

1995 Pontiac Grand Am mileage: 125,000. Our car overheats when driving slow or sitting at a stoplight, yet the temperature gauge is cool when driving 60mph hour or more. It has the quad 4 engine. Since January different mechanics have replaced three thermostats, two radiator caps, flushed radiator and put new tanks on the side of radiator, and the timing chain. We are at a loss. Every time we take it to a repair shop, and explain what we have done to it, they assure us they can find the problem and lo and behold, we pick up the car to find another new thermostat put in. No one is solving this problem. I am wondering if it is a cracked head gasket or heater core, but my main concern is how do I get a mechanic to find out what is wrong so we can fix it. I would be thrilled to have an honest answer. Thanks in advance.