1999 Concorde LXI 3.2L Engine with 150,000 miles

After driving my car for about 10 to 20 minutes my car starts spilling the coolant thru what I think is a breather hole in the reserve tank. The temperature gauge doesn't marks the engine as "hot" when the car does it, though after it does it starts overheating. I replaced the thermostat as I thought maybe it was stuck on closed since the coolant was coming back to the reserve tank boiling hot. The radiator cap is ON the reserve tank and there is no other way to fill up the coolant, but thats not where the coolant is spilling from. I have a little device that measures temperatures with a laser and after driving my car for a few minutes I tried it and it said that both hoses coming in and out of the radiator were about the same temperature 190+.I found unusual that the thermostat was on the hose that comes from the radiator to the engine, on the bottom right behind the alternator, rather then the one that comes from the engine to the radiator, unless I'm mistaken and it flows up thought I don't think that makes any sense. There is a valve to " purge " the air in the engine, its on top where the hose from the engine goes to the radiator. Some times after it spills all the coolant I fill it up and it works fine for a bout a week or so then it does it again.I took it to the mechanic twice and they don't fine anything wrong with it, since it appears to work fine while they have it and continues to do so for a bout a day or tow after I get it back, but then again it starts acting up.I really don't have a clue any more and I would really, really appreciate what ever help you guys could give me.
Thanks so much.