Diagnosing a random overheating issue.

98 Subaru Legacy L Wagon. (2.2)

Back the 22nd of December (2015), I had an overheating issue after a solid 2 hours 15 min of highway driving for about a 3 hour trip. I stopped, tapped on the thermostat housing and put a few (16 oz) bottles of water in the coolant reservoir. Didn't make it a half a mile and the temp came right back down to normal (So I figured it was a T-stat issue or not enough water) and have had no issues until a few days ago, which would be a little over a month. Driving the other day, it happened again. I was fairly close to home (about 4 miles), So I decide to limp it home and few times the gauge started to go down a bit then crawl back to hot again. After a few minutes of that, I cranked heat full blast and it came down a bit to where I could make it the rest of the way home. When I got home, I left it running and checked the reservoir, it was full. The hoses to/from the radiator were not hard and I could open the cap to the radiator without issue. (despite the overheating only a few minutes ago.)

I haven't had an issue since I turned the car off (just like the first time it happened, a month ago) after I got home and gave up, but used it later that day and the 2 days following it without issue. Today I had some time to spare after work and before it got dark. so I figured I'd do a little investigating. In park, I brought the engine up to 2K RPM and held it there for 5 minutes or so and didn't see the needle creep past normal levels. The hoses didn't seem to have pressure again, but I also didn't see the fans kick on either. I haven't noticed any puddles under my car, but the reservoir is empty and the radiator is still full. Pulled the dipstick and the oil is a little low (not surprised, being it leaks a bit), but looks normal for needing to be changed in about 3 or 400 miles. Even if the oil wasn't chocolate milk looking (which I didn't see), I would suspect that if coolant is getting into the crankcase, the oil wouldn't show low, right?

So I'm not sure if it's a gauge or sensor issue. T-stat issue. external coolant leak or internal leak/head gasket. A water pump or it's simply just air bound. I'm not overly concerned about it, being I don't plan on going far enough anytime soon to where I can't limp it home if the issue repeats, But I would like to have a good idea of what the problem may be so I can fix it or get a new car (if the engine is a catastrophe due to a head gasket issue.) So I would appreciate any input or advice from you guys. Thanks!