My coolant is leaking and car keeps stalling - for 3 years now!

My usual mechanic is Firestone Complete Auto Care. Im loyal to them and only go to them. Ever since I obtained my car in 2009, the car has stalled at least 5 times. It happened about a week ago. Before that it happened in September 2011. Before that it hadnt happened for over a year. When it happened last week I realized I also had no coolant even though I got the coolant filled up 2 weeks prior. I dont know if there's some correlation between the coolant and my car stalling. Anyway, as for the coolant, it has gotten worse over the last year. I went from putting in coolant every month, to every 2 weeks and now it looks like, after putting it in less than a week ago, now im gonna have to fill it again. When I went to Firestone in September 2011, I spent 500 dollars for them to change the "coolant gasket" and the thermometer and one other thing. That was supposedly supposed to fixed both problems im currently having. I hope I gave more than enough informations and would appreciate an answer ASAP. Im bringing it back to Firestone tomorrow but this time, I want to go with some suggestions on what they sure REALLY looks at.
Thanks
Melissa