GM-GMC: Intermittent Overheating, 2003 gmc savana, gmc savana

Question
2003 GMC Savana overheats occasionally.  Turning on heater brings temperature down, usually to normal.  Other times, temperature normal.  This happened last summer and had thermostat replaced, which fixed problem.  Is it likely thermostat again?  If it is, is there some other problem like sludge that is causing thermostat problems?

Answer
It is possible that the thermostat is faulty, however, when they malfunction they usually stick open, not closed, this would give you over cooling, not heat inside the cab etc.  If it fixed the problem last time, it is well worth, trying again.  I recommend usuing a GM thermostat.  One other thing you can check is your rad cap.   If it doesn't hold pressure, it can cause an intermittent overheating condition.  I had a sunfire given to me because the owner had a garage tell her the engine was no good because it was overheating, after they replaced the headgasket, head, water pump, thermostat, everything they could think of.  All the problem was, was the reservoir cap wouldn't hold pressure and the coolant would boil causing the engine to overheat.  I found a cap in my toolbox, threw it on for free, and gave the lady her car back.  It is something that is easy to replace, relatively cheap and is often overlooked.
Hope this helps.