2001 Hyundai Elantra Temperature gauge and sender

"The thermostat shouldn't fluctuate that much." I bought the car a month ago. Had everything checked on it, compression, coolant pressure, leaks, everything. It was in great condition and ran so-o-o-o smooth. The temp gauge would fluctuate between just below half to just above half, so it wasn't that much. One day it overheated suddenly. My mechanic could only guess that the thermostat had stuck closed. He said it looked like someone had done a "Mickey Mouse" job of installing it, with epoxy sloppily put on it. He replaced the thermostat but the damage had already been done. That being so, the temp would rise to 3/4 then back down, it was erratic. The check engine light stayed on all the time, and it ran rough. One morning it stayed near cold for ten miles, the next day it would threaten to go near the red. I added 1/2 bottle Bar's coolant sealant and it really improved a lot, almost normal. The check engine light went off, and it idled and ran smoothly. The temp gauge would move around halfway + or - a bit. Last wednesday I took it out, stopped at a store, and it wouldn't start. The temp read below halfway. I put a friend on notice that he might have to come with some plugs and tools, but wait until I called after I let it cool down. When the gauge said it was completely cool, it started right up, no cough, sputter, didn't have to give it gas, was a perfect start. I drove to an automotive store, bought some more sealant, but didn't put it in yet. Waited for it to cool down, and it started right up again. Now I noted that the temp would always rise to just below halfway and never move again. Since then that is exactly how it has worked ever since. It always starts at cold, moves up to the exact same spot on the gauge, and does not move even a hair no matter how hot or cold it is out, running, idling, A/C on or off. You may say that it shouldn't fluctuate much, but my (short) experience with the car says that it normally does fluctuate SOME. I don't feel good about driving the car around with a blown gasket, but I would feel a lot better if I could keep an eye on the temperature so I could drive or stop accordingly. What amazes me at this point is that the plugs haven't fouled yet, or if they have, they magically clean themselves by the time the engine cools down. Maybe the moisture evaporates, I don't know. Every cold start is perfect. So back to the temp gauge question. Is it worth the trouble or do I take my chances until I get the gasket replaced? I don't think that I'm up to taking the dashboard apart if you think the gauge is sticking. But if it could be the sender, is that hard to get at and replace? Thanks for your help.
Paul