Dilbert

Have a 2001 LL Bean 3.0 H6 OB with some overheating issues. Been fighting this for some time and at some enormously growing expense. FIRST: Replaced radiator with new aftermarket because bottom half of radiator was "Cold" & "Must be 'Plugged' up"... Also changed cap and thermostat w/ new OE Subaru parts. Also ran pressure test for several hours; no drop in pressure, no leaks. Some improvement, but sporadically overheated, filling overflow, and overflow would not siphon back overnight. No OBD codes, no lights on dash. SECOND: Replaced all three catalytic converters as they were shown to have "Too High" a temperature differential between front and back of the cat. Some improvement to performance but not any change to overheating. SYMPTOMS: Primarily a problem when climbing hills at freeway legal speeds. Around town driving: no overheating, but every morning replace 1/2 to one cup of coolant; sometimes from overflow expansion tank with turkey baster, sometimes [about 50-50] have to add coolant to system [both radiator and overflow] Sometimes it's more like a quart low in radiator. I'm in Sacramento, CA so no Mt. Everest climbs... mostly under 3500 ft elevation on major highways. Driving around trying to keep it under 1500 RPM is getting old. Most hill climbing is in the 2500-3000 RPM range. Could the 2-stage OE muffler be a cause? BTW Both fans operate correctly and currently about 185k miles