Temp. gauge drops; engine wants to shut off
 

I have a 2001 Chevy Blazer LT with the 4.3L V6 Vortec, and she's been acting up lately. I figured I'd post my issue up here before I start tearing it apart and dropping a lot of cash.

This started after I replaced just about every sensor and valve plus a bunch of other stuff that I could get my hands on because it didn't want to start and stay running. Anyway, it doesn't happen all the time, but while I'm driving, the temp. gauge will start to drop, causing the voltmeter to do the same and the engine threatens to shut off, and it has a couple times, but it'll start right up afterwards. I checked the coolant level and it's a little fuller than it should be, and I can't find any visible leaks anywhere. I don't smell coolant, and when the gauge starts to drop while I'm driving, I give it some gas and it's fine. Not quite sure what's going on and I'm pouring through my Haynes manual, trying to narrow it down. Possibly a bad thermostat?

I need some help because now my husband's 1999 GMC K1500 Suburban is overheating and the oil pressure is dropping, so his is out of commission right now.