1999 Subaru Outback Car Overheats - sometimes

Back in the summer I got an oil change. At that time the mechanic said something about hoses - radiator - and I should have listened better. In September we noticed that it would begin overheating due to loss of coolant. One occasion my family was in the car and the air was on as I went in to do a business transaction, when I came out I was told that my air "sucks" - it wasn't blowing cold air. The temperature gauge was where it should have been. Some man came to my window and said I was leaking a lot of fluid underneath. When I was driving off, two other dudes said - look at that antifreeze! - Then they told me to keep an eye on my gauge. I went home about 20 miles and all was well. My husband kept the coolant topped up since I was losing about a cup a day. If he neglects to do this, then I will tend to go staight to "H" and must turn it off. This happened in October as I was sitting at a light. It flooded out and then I restrarted it and it went to a safer place to call for help. Basically the coolant level will get so low that it will have some sort of eruption. First took it to Midas where they said all it needed was a new cap. That wasn't it. We then took it to a local mechanic who discovered that because of the previous overheating the water pump had begun to leak and it was replaced as well as the timing belt. Less than 100 miles later on the coldest day of the year, I had let the car run 10 minutes before driving off. I got about 14 miles and noticed there was no heat - it was all cold air and then noticed that the temperature guage was at H again. It was then towed back to the shop where they decided that it was a failed head gasket and recommended that we have it towed to the local Subaru dealer (Dryer & Rinebold). They could not find any leaks and didn't feel the head gasket was faulty. Still the vehicle continued to lose coolant. On Christmas Eve I took it to another local shop where they diagnosed a radiator leak that only occured on warm up and cool down and did not leak on to the ground. The radiator was replaced. I noticed when I got home (15 miles away) there was still the sweet smell of antifreeze upon turning off the car. The next day I drove the car about 20 miles away and got out and looked underneath the front of the car and noticed antifreeze dripping. Each driving episode of about 60 daily miles - we must add coolant. No evidence has been found of coolant in the oil or vice versa. I have now spent over $1400. The overflow tank fills up. Is there some kind of pressure build-up in the system that is causing coolant to overflow out the cap or? Please help!