Cadillac: 1999 Cadillac Seville SLS Overheating?, blown head gasket, cadillac seville

Question
I purchased a 1999 cadillac seville sls last november.  One day while driving home on the highway in mid april The temperature gage went from the middle, and ticked up to the H and my car then beeped and side engine hot/idle engine.  I then went back on the highway a week or so later and the same thing happened, so I pulled over and let it cool down then left and it ticked back down to the middle.  Now since then when on the highway or driving around town for a while, the temperature sensor will tick up to about the 6th mark, the 8th mark is Hot.  I'm not sure if its over heating or what? I took it to a GM service shop and they couldnt find anything wrong.  No blown head gasket, coolant is fine....the temperature gauge would not raise higher than the middle when they drove it.  I have taken it to 3 places so far with no luck!  When it ticks up it usually ticks right back to the middle line between cold and hot pretty quickly, within a minute or two, but there has been those couple times where it went straight to the H and i had to pull over and shut off the engine.  I also sometimes have loss of power, as in when I accelerate it wont accelerate like it should.  I will push the pedal to the floor and it will just slowly increase speed when it should jump the gun. Any ideas? Thanks!

Answer
Next time it does get up to the Hot side, pull over and open the hood and see if BOTH cooling fans are operating.

If they are, I would install a new thermostat and see if it clears up.

Without being able to have access to the vehicle to perform tests this is not going to be an easy one to diagnose via this method.