Overheating almost - 96 Auto 111K

Hey there!
My coolant guage on the dash showed that it was in the red yesterday on my way home. I cranked the heat to full and the needle dropped to its normal level.
I made it home and threw in some more coolant into the resevior hoping it would help.
The next day, I tested with the heat off and the needle worked its way to almost into the red again.....

-3 yrs ago, I replaced the thermostat b/c I was getting lukewarm air when the heat was on.
-can't remember when I had a coolant flush last.

Any suggestions for fix?

Thanks S

do a complete flush on cooling system... Pull out thermostat.
1) Drain it 2) fill with water drain it. 3) fill with water and then Prestone Flush. a run the truck for about 15 minutes with heat on.. Then drain it. 5) fill with water then run again and then drain it. ) install new thermostat ( factory recomended heat range) and then fill with 50/50 water and coolant.
You can also do a backflush..which is flowing water from a hose in the opposite direction of the coolant flow. My radiatior is from top to bottom. So then I would disconnect the top hose and fill from my resivoir tank to the bottom of the radiator. This will clean out the buildup. I would do this after step 3)

I just did it to my truck..and its a 2002. At one point i just disconnected my top radiator hose and put a garden hose in the resivior tank and let it run. It must have let out sediment for at least 15 minutes. I let it run clear then did the Prestone flush. . I think there is a flush that you can use for longer than 15 minutes. I think you use it for a day or so in normal driving. This would help with heavy deposits in cooling system. Again all of these are added to just water and no coolant at all. Now my truck warms up quickly..maintains temp even after I leave it for a while. the needle never passes just under the halfway point. A thermostat that runs to cold is not good eithe .Try to get as close to recomended temp as possible