91 century stalling and radiator problem

I have a 91 century, and whenever... well not everytime but it could happen back to back one day or none at all... I go to slow down, my engine will just stall out. No jerking or anything of that nature happens. I also just acquired this car and so I wanted to see if I needed a tune-up since I did not know when this car had one. So I took it into this shop around here last tuesday. They said everything looked fine as far as needing a tune-up, but they put the car on the diagnostic machine. It took them a little while to figure out the problem, but they said they finally figured out I need a torque sylonoid converter. Now I haven't done anything with that though yet. I had to go directly from the shop to my work. Well, on my way to work... the temperature light started coming on. This had never happened before... and it came on and went off twice during that little trip, about a 6 minute trip. Yesterday I checked out my coolant tank, and there was nothing in there, and so I filled it up with the needed solution. I checked it a few hours later, and it was still at the proper level. I then went to work again, checked it, and it was still good. But then just today, I'd say this would be like 17 hours later, I checked it and the tank was completely empty. Another thing is, since I got it back from the shop, it has not stalled out once... I got it out tuesday afternoon. :Can anyone offer any advice/suggestions on what is going on? This whole thing just sounds kind of messed up to me... but I don't really know much about cars.

I have a 91 century, and whenever... well not everytime but it could happen back to back one day or none at all... I go to slow down, my engine will just stall out. No jerking or anything of that nature happens. I also just acquired this car and so I wanted to see if I needed a tune-up since I did not know when this car had one. So I took it into this shop around here last tuesday. They said everything looked fine as far as needing a tune-up, but they put the car on the diagnostic machine. It took them a little while to figure out the problem, but they said they finally figured out I need a torque sylonoid converter. Now I haven't done anything with that though yet. I had to go directly from the shop to my work. Well, on my way to work... the temperature light started coming on. This had never happened before... and it came on and went off twice during that little trip, about a 6 minute trip. Yesterday I checked out my coolant tank, and there was nothing in there, and so I filled it up with the needed solution. I checked it a few hours later, and it was still at the proper level. I then went to work again, checked it, and it was still good. But then just today, I'd say this would be like 17 hours later, I checked it and the tank was completely empty. Another thing is, since I got it back from the shop, it has not stalled out once... I got it out tuesday afternoon. :Can anyone offer any advice/suggestions on what is going on? This whole thing just sounds kind of messed up to me... but I don't really know much about cars.
First welcome to af, now, the cooling system should be pressure tested to find the leak, second ask the shop if they disconnected the tcc solenoid for test purposes, post back what they tell you.