Why is our 2000 subaru forestor's temperature up sometimes and not other times?

I would suspect air in the system, a bad thermostat or cooling fans that are not working.

I initially suspect F_O_R's door number 1. When a migrating gas bubble sits adjacent the coolant temp sensor it'll spike temporarily. Elevate the front of the car a bit and have a careful purge done before spending more on parts.

Thank you for this information. One other concern is that when I accelerate the temperature will go back into the normal temperature range but also when I am at a stop light, In park, and rev the engine without moving, the temp will drop back to normal on the gouge.Your thoughts on that.

Perhaps the water pump is worn and not circulating the water as it should.

Subie's WPs are pretty golden, so I'm still with yoiur Door Number One. 96nyx: you mistyped "normal" twice; regardless, the temp gauge "pumping" is because of trapped air in coolant. Bleed the system carefully. If it won't stay bled check for outgassing because of blown head gasket. See other long threads for procedure.

Thank you for your solutions. The head gasket was changed, the water pump was changed, and the thermostat was changed twice, as well as the radiator, and cap, and the fan was working properly. Prior to the repairs being done the Temp gauge was fluctuating after warm up and driving as well as after repairs. I stupidly added head gasket sealant and followed the direction meticulously,before the repairs were made. One other symptom is I hear a 'gurgling' sound behind the A/C vents in the dashboard even without turning on the A/C. After repairs were made a po 443 Evap control solenoid error code kicked out from a check engine light that came on after repairs were done. Not sure if the last two items are pertinent to the cooling system problem. Thoughts on this. Thank you.

Gurgling sounds = air in the system.