1992 Toyota Celica Overheating

1992 Toyota Celica 2.2, 172,000 miles, manual transmission.
****Prior to this I've taken several road trips (320 miles round trip) and had no issues aside from a squeeky belt which was replaced.****

I have had the distributor replaced, (the car wouldnt start period, by popping the clutch or jumping) and on driving it home that day, the car overheated, I stopped refilled the radiator and it ran fine for about 3 weeks now. Recently the car has started overheating while traveling distances less than 4 miles at normal traffic speeds (35 to 45 mph), on parking the car, you hear a gurgling noise and a small amount of coolant coming from the overflow bottle. I have had the radiator flushed 3 times recently, and still nothing in the way of progress. Both the air conditioning and the heater work fine, however the car overheats in an expedient manner while running the a/c, and will steadily overheat (slowly) while running the heater.

I'm curious if this is mearly a thermostat problem, or could this be a waterpump. I'm taking it in on Monday to be looked at, however, if it is the thermostat, how hard is this to replace, and is there somewhere I can find instructions to do so. I was also looking for a second opinion from someone here before I took my car back the mechanic that did the distributor repair.

Thank you,
Charlie