Runs loud and sluggish when warm/hot. 350

Hi,
I have a '94 full size blazer. 4x4 350ci.
*sometimes* when I turn my truck off and then, within a few minutes, start it up again, it runs much louder and with much less power. After some running the engine noise dies and the power comes back.

This has also happened after sitting in traffic (and it's hot here in texas).
My temperature guage seems to be working and does go up but very very little.

If I had to guess I would say that it's running hot but the gauge does not indicate so. I think the guage works too. I can turn my engine off but leave teh key in teh acc position and watch the engine temp go up - so i assume it works.

Thanks in advance!