98 Subaru Outback overheating?

My 98 Subaru Legacy Outback 2.5 -no turbo- is overheating in cold weather-not super hot, just 2/3 hot. I have had the engine rebuilt and don't suspect head gaskets? They already blew. It only heats up after 30 miles or so of highway driving and the heater doesn't get real warm now. The collant levels are adjusting normally and when I pull over to idle, she only takes 5 minutes or so to cool down. Made it across country fine this summer with a load, why now that it is cold is the car overheating? Are the radiator and hoses easy to change by myself and where to find parts?