Engine Cooling problem
1999 Subaru Outback 4 cyl All Wheel Drive Manual 130000 miles

For a year my 1999 Subaru GL outback has been overheating occasionally -- will happen 3 days in a row and then not for a month. Sometimes when this happens, and I turn the heater on to cool it down a little, the heater only blows cold air, or blows cold then hot then cold then hot. But 75% of times it overheats, the heater works ok. Thermostat was changed and radiator flushed, which didn't fix it. Waterpump was replaced, and then no problem for about 6 months, but now it is happening again. I don't know if it has anything to do with it, but the air conditioning hasn't worked in 2 yrs. The power is fine, including on hills. 3 different mechanics haven't been sure what the problem is. One is suggesting a new radiator.... Any