Overheating (not TOO serious)

Hey there. I just recently purchased a 1986 Subaru GL Wagon, with a 1.6L carburated engine (it's got 170k+ miles). It works perfectly besides one problem; after driving it for a while (about 20 minutes) the temp gauge will go high (a needle's width below the red section). I turn the heater on and it puts it down a bit (not to a comfortable area though). I read various forums, etc, and tried troubleshooting a few things. I looked to see if the fan will switch on, and it runs. The coolant levels are perfect. There seems to be no gunk ON the radiator, and I had the coolant flushed today at a mechanic. I'm almost positive it's not the water pump, as the temperature goes down when I turn the heater on. I did notice a gurgling sound a few times from under the dash which might suggest the water pump is sucking in air? I also replaced the thermostat. I checked hoses today; in fact, after having my radiator flushed, I TRIED to overheat the car, and upon getting to the top of this hill, my upper radiator hose blew. I replaced it though. This car problem isn't too much of a problem when it's colder out (because I'm prone to have the heat on), but I would like to travel a bit and I need to fix this. What is going on?