I will start with the basics here first, i.e. Your cooling system needs to be properly filled.
A low coolant level will give you peaks and valleys in temp readings because the sending unit is not in constant contact with the coolant; it is just splashing on it (like dipping your finger in and out of a cup of hot coffee). For best results the sending unit needs to be in constant/direct contact with the coolant to measure its temp.
To fill your system:
Start with a cool engine and have everything you may need ready i.e. funnel, coolant, helper to rev engine.
Try revving the engine up to 2000 RPM's while topping off the radiator, then put radiator cap on while still revving engine. Return to idle.
Make sure your overflow tank is not too low either.

If that is not your problem then let's look deeper.

What color is your anti freeze? Factory "orange" is not good in my opinion.
Orange coolant can gel-up and clog your heater core.
You may need to purchase a "cooling system flushing kit" or pay to have it flushed out.

Do not reuse orange coolant. Revamp it with either the old green stuff or ask your parts man what's best.

This happened to my 1996 s10 blazer; turned out to be a restricted heater core.
My heater made a gurgling sound at certain rpm's, and did not heat well at all.

I flushed it out (the whole cooling system) following the instructions on a acid based flush kit and refilled it with the green coolant.
After the flush the gurgling sound was gone and I could not believe how much it improved the whole system; especially how hot the heater blew.

My blazer came with the orange coolant from the factory.
I was told if your level gets low with orange, it will thicken up.

I would also check for any kinks in your heater hoses just as another possibility.

Good luck!