List of "Could Be's..."

Air trapped in head. Cold engine: remove thermo-air valve from top of thermostat housing. Fill radiator until fluid comes out of housing. Pop thermo-air back in. Idle with radiator cap off until flow is present and look for oil globs or fine air bubbles (indicating leaking head gasket).

Bad Pump. Check belt tension. My alternator bracket broke a bolt and was bouncing free so the pump would grab sometimes and not others. Temp would shoot up and drop down over and over.

Leak on intake manifold. Not obvious. Coolant runs through the galleries of the intake and the throttle body (helps warming things up).
Easier to spot at night with a flash light if it's a little drip, or spread out a cheapy drop cloth with a paper face under the engine and leave the engine run for a bit. Some things leak when they warm up. Others when they cool down.

'Course it could really be you have a bad head gasket too.