Leaking anti-freeze

I own a 174,000 96 Sable 24valve dohc. I have a anti-freeze leak by the right front wheel well after I use the air conditioning. I will use the air and when I turn the car off, anti freeze will leak out of the engine from somewhere in the right front of the engine. I have had the corroded metal pipes that the previous post talked about and changed them along with the thermostat and anti freeze reservoir a year ago. When I do not use the air conditioning, the car is fine, and when I use the a/c it is hit and miss with the leaking (usually does it when it is 80degrees or more out). The car is not running hot at all. I did notice when I stopped it once when the low coolant light came on, there was boiling anti freeze to the top of the antifreeze reservoir. I was hoping you can tell me that the thermostat is bad, or some other cheaper solution because I read that it could be a head gasket problem, and that repair is probably more than the car is worth. Thank you very much for reading this and your response