at 65-70 miles per hour my engine rpm seems to fluctuate almost like a big wind just hit the front of my truck temporarily slowing me down and then speeds up again. It seems to be related to the engine temp on my dash gage. should I change my thermostat? My truck has 250k miles on it.

 

65-70mph doesn't put much of a strain on a nearly empty truck. And the temp gauge shouldn't fluctuate very much, if at all. Mine tends to be pretty rock solid on a steady cruise. Even the temp reading on the Outlook is steady.

If your gauge needle is wavering and the truck is surging, I'd be looking for an electrical problem first.

 

Don't sound like Temp related. Sounds like fuel delivery. Maybe getting a little air in the system somewhere. I'd check the fuel filter and make sure its tight.
If its a second Gen truck the out of the tank fuel pump could be messing up.

Can't get more specific, not knowning what year your truck is.

Dave

 

Sure it's not the fan clutch coming on? How high does temp go? Coolant level good?