It sounds like it was a bad tstat, since now it stays in a temp range instead of overheating after a short drive. As for the leak, I get the feeling that little hose isn't supposed to have coolant going through it.
Did you replace the tstat housing gasket? If not, you're gonna kick yourself for it. If you did put a new gasket in there, I'd try tightening up the housing just a bit more, not hulk smash kinda tight, but make it difficult to get it back off. I'd replace the TB gasket while you're in there, as well as the water pump. The whole rough idle and lack of power could be a vacuum leak. And a new water pump is always a good idea when you're having overheating issues with a GM V6.
Honestly, you'll spend less fixing up this GP than you would on most other "reliable" vehicles... Parts are usually really cheap and there's still plenty in junkyards to pull the hard to find bits from.
