Hi all, I'm new here and just bought a 97 outback and have been having overheating issues. A mechanic did a leakdown test and determined that it was a leaking radiator, so I replaced the radiator along with the thermostat, and the upper radiator hose (the aftermarket lower hose didn't fit). The engine runs great and drives fine around town, but when I take it on the freeway it overheats. It'll run well for anywhere from 2 minutes to an hour until the temperature suddenly rises. It takes about 30 seconds to go from normal temp to red line. If I stop for a minute and run the fan then it will return to normal temp and I can drive another couple of miles before the problem develops again. I took it to another mechanic who did another coolant leakdown test and he said the system held pressure just fine.
Any ideas? I'm worried that it's a head gasket, but shouldn't that show up in a leakdown test or wouldn't it also overheat at low speeds? Could the thermostat just suddenly close?