1996 Mazda Astina 1.8 Overheating issues.

I have a 96 Astina. The engine seems to be overheating from time to time. The Temperature gauge goes up to about 3/4 then back down to 1/2. It does this a few times until eventually it gets higher and higher and then goes into the red (But back down eventually) All of this is while I'm driving.

Just a quick history, I had replaced the head gasket about 2 months ago. Changed the oil as well (Problem was before oil change)

I checked the Oil & Coolant levels, All ok. I had the cooling system tested for leaks - No Leaks were found.

Now, The question could it just be the Thermostat causing this, or is there something else that might be wrong?

Thnx in advance