Subaru - Outback :: 2001 H6 Overheats Only When Drive For A While On Hot Day

My 2001 Subaru Outback H6-3.0 is at 215k miles (I bought it for $4k at 201k miles). It overheats only when I drive for a while on a hot day. I was trying to get to Yosemite with my whole family in the car, and rather than do the intelligent thing and go back home and get it repaired, I kept going, stopping when the gauge started going past the middle to let it cool down and replace any water it was missing (after the engine cooled, of course). Amazingly, we managed to use the car almost normally during the trip, as long as we did short-ish trips and kept some water in the car. Having to pull over randomly isn't so bad when you are in Yosemite anyway.

There was a mechanic shop in the little town we stayed in, but they weren't equipped to to do a head gasket job in the little time we were going to be up there, and we needed the car to get around anyway. So I tried putting some of that coolant leak repair gunk in there and it seemed to work. We made it the hundreds of miles home, no problem (but it was downhill, where the trip up there was uphill). Problem solved, right?

Well, about a year later of only driving the thing about 50 miles in any given run, and not terribly hot weather, we decide to go a few hundred miles south. Sure enough, it was a hot day and after about 60 miles I saw the temp gauge go past mid and immediately pulled over. After it cooled a bit, I drove it (in two hops, because it started heating up again quickly) to a shop. They tested it and sure enough it's a head gasket leak. They said it'd be $2500 to fix it.

So far this car has been pretty good to me otherwise. Numerous times, I've taken it up 300+ miles into the snow and ice covered roads to go skiing and LOVE not having to put chains on. I am guessing that all the cold air up there kept it from overheating while I was pushing it pretty hard up some steep inclines without issue. It seems like there is a temperature threshold beyond which it starts to quickly overheat, and below witch, the gasket is holding fine, or something along those lines.

So, given that I like this car, is it worth another $2500 to get her back on the road? Should I just try the leak stopping junk again? Should I have been using "SUBARU cooling system conditioner" this whole time, even though it's not one of the supposed head-gasket-problem engines? Should I just drive it until it finally overheats to death? It's probably still fine to drive in moderate to cool temperatures (winter is coming), but I haven't driven it since the leaking head gasket diagnosis. I don't know if I did permanent damage to it that day, or if it's just its usual self still. It didn't get into the red, heat-wise.