Rough idle rapidly alternating between 400-800 RPMs, possible blown head gasket

Starting today I got a very rough idle where the RPMs are rapidly alternating between about 400-800 RPMs. The car isn't quite getting low enough RPMs to stall out but it's getting close. When it's actually driving then I don't have any symptoms like this, only while idling. Years ago, I was told the head gasket is leaking, and I've had a small loss of coolant that I've had to periodically refill. Occasionally I've had a similar issue where the RPMs bottom out to 400 RPMs and jump up and down a few times, and then it would stop. Now it's persistent though while idling, it never seems to want to smooth out. It's not the same as a misfire as it only happens while idling. If it was caused by coolant in the cylinders though I'd think there would be bogged down acceleration and other drivability problems, which I didn't experience that I know of.

I have had some slight overheating issues along with the loss of coolant, but never to the point of it becoming a problem. Generally, if the coolant is low the temperature needle would start getting a little over the halfway mark, which was usually my tip to refill it. This has been happening some lately as well. Again, not to the point of substantial overheating though.

When I had a similar rough idle issue in the past it was because the throttle body was bad. I replaced it with a good working used throttle body in 2020 and never had that problem again. When I was having throttle position sensor issues causing idling problems, it was causing it drop to 400-500 RPMs and staying there, not rapidly alternating like it is now. So, I'm leaning towards either throttle body or head gasket issues. What do you guys think?