1998 starting problem

I have a 1998 Monte Carlo. I have had new wires. plugs, fuel filter, #4 fuel injector replaced, and they cleaned the egr valve. I have put in around $1500.00 the last 6 months. the service engine light went off. But they can not find out why my low coolant light stays on,not low. does not over heat either. Temp gage goes to just below mid point. eventually it shuts of about 20 minutes after the car has been running. My new problem now is after driving the car for about 15-20 minutes and you stop it, it does not want to start, or if it does start it chucks and keeps killing. I hate to keep taking it back into the mechanic and spending more money and the problem not getting solved. They have done around 4 diagnois they come up with nothing. One person said to change the egr valve, another says the sarting problem could be you theremostat. Anyone had this same problem or know what it could be.

thank you