2001 Tahoe Feel like misfires at high way speeds and up hills

Used 01 Tahoe with high mileage, I have had it for 3 months and have driven it between WA and MN (1700 miles 1 way) about 3 times. Its been running great until within the last 3 weeks it started to stutter or misfire when I would hit 50-60 mph. I would give gas to power through and it would power through with no other noticeable problems until I dropped to a lower speed or hit a hill then I would need to power through again.
Ive heard from friend it could be anything from plugs and wires to blown head gasket to water in the gas tank to deeper engine problems.
A little over a week ago the temp gauge would rise to 235 and it took a little longer to heat up. I added 50/50 mix coolant (friend believed it was a possible air pocket) and the heat returned to mid point of 210 and the heater performed normally again.

I havent seen smoke of any kind no indicator in the instrumentation and I have added heet and gum (not together but at different times) to the gas tank.

When I bought it it came with larger exhaust pipes and I have seen water drip from the pipes from time to time even before the above issue. I havent smelt any sweet smells from the water in the pipe Is this a obvious problem to anyone else out there?