98 Chevy Silverado 2500 overheating problems

I have a 98 Chevy Silverado 4x4 V-8 I am not sure what size the motor is exactly. Last week my truck started to overheat, it didn't get to the red mark but it was very close and the check gauges light came on, also while it was doing this I noticed I didn't have any heat at that time. We bought a new thermostat and a week later it happened again but this time it did get red hot. So I assumed that maybe this happened because the thermostat had gotten clogged due to rust particles in the radiator because the coolant was a rust color and seemed kind of gritty. So today I flushed the radiator and then once again bought a new thermostat and this time I drove it a short distance and my temp gauge seems a little fickle. It is moving around going from a few notches above 100 to a notch below 210, I am not sure what to do at this point as I am afraid that it is going to overheat with me again. Any suggestions would be great, thanks :)

Mine did that after I flushed because it seems there was still air in there.
Now, when refilling I turn heat on HI & let it run till thermostat opens &
keep filling. There are more elaborate techniques also, but that was
enough to work for me. Now, goes to 200, opens & locks at 190 like
it should.

I will try that to see if it works, I am afraid that if I drive it to work I might get stranded again....lol Hopefully that is what my problem could be.

Ok, I let the truck run with the radiator cap off and when the thermostat opened up I added more coolant because it was a little low. It doesn't seem like it is going to overheat. But the temperature guage is still bouncing around, it stays right under 210 though. I have no idea why it would still go up and down like that.

The needle bounces around because there is still air in the system. It may take a while to get it all out. Keep an eye on the coolant recovery tank and refill as needed when cold.

Right, i let mine run for about 20 minues & kept letting it cycle & filling.
Some people have said raising front end helps but I dont know.

Today I let it run for about 20 minutes with the radiator cap off and filled it up as needed. My driveway has a small slope in it so I had the front end of the truck up on it so I hope that helps too. I haven't gave it a test run yet and I am gonna drive it to work tonight and see how she does. I'll keep ya posted! Thanks :)

The truck did great, it didn't overheat at all. But I noticed if I don't let it warm up before I take off in it then the temp runs a lot warmer than it usually does espeacially if the oil pressure is higher than normal.