Cadillac Repair: 1997 STS Thermostat Replacement, blown head gaskets, coolant level

Question
QUESTION: Hi.  Just got towed home after my 97 STS overheated despite the coolant level being full.  I suspect a bad thermostat.  Is this easy enough to replace on my own?  Where is the thermostat -- upper or lower hose at engine?  Is it accessible or should I go to a repair shop?  Thanks for any help.

ANSWER: At the upper hose where it attaches to the engine.

---------- FOLLOW-UP ----------

QUESTION: Actually, found the thermostat at the end of lower hose.
I replaced it and filled coolant and everthing seemed fine for about a half hour.  Then while on the highway the car overheated again.  Any ideas?

Answer
My apologies on the thermostat location. Was thinking different engine.

Now that I have the correct engine in my head, The most common problem with this engine is small leaks in the head gaskets that allows combustion chamber into the coolant passages and stops the coolant flow.

have the engine tested for blown head gaskets as this is very very common.