[97TT6SP]

I would flush it with DI water like 5-7 times until nothing comes out except water. What I mean by flush is drain all old fluid out, fill with DI, and drive around a bit to heat it up and circulate it. Be carefull though because it's tough to fill the system all the way until the thermostat opens. I think you can even buy an additive to help dissolve anything that is coating your radiator or causing any blockage and hence causing a hotter than normal temps. I agree that the thermostat should also be checked too.

Are you losing fluid over time? It might be a leak. When my water pump was slowly leaking a had similar symptoms until the level dropped enough such that the temp shot up all the way.

 

[97TT6SP]

Not sure, I just used about 10Gal DI water and flushed 5 times. Just do to the autoparts store and tell them what you are looking for. I think it's just a little bottle that yuou add to the water. Rinse your system a few times with water before using it though. Like I said before, you can't fill up the system all the way until the thermostat opens which requires you to run the car. I would fill the system and run the car with the radiator cap off until the thermostat opens. The level will drop in the radiator when it happens. Then add more water (put the cap on!), drive around a bit (kinda like swishing mouth wash), drain, repeat until the water comes out clear. Don't burn yourself!

I hardly doubt that mixing two kinds of coolant would cause precipitation (or sludging). You might want to check the quality of the coolant by buying a "coolant meter" which just measures the density of your coolant.