Suburban :: 1999 Only Runs For An Hour At Highway Speeds

Ok so this is something I can't quite wrap my head around. I have a 1999 K1500 Suburban, 5.7L. I have only owned it a few months. The thing is, shortly after I got the truck, we took it out of town, where it died after a little over an hour of driving at highway speeds. I had punched the odometer to watch the gas mileage, and we had run it about 70 miles when it died. We got it to our friend's house, where we were headed anyway, and stayed over, planning to look at it in the morning because it was later than part stores stay open in our rural area. Thing is, in the morning it fired right up. So we headed home, and once again, after about another 70 miles, it shut down and refused to crank.

This time it happened as we were coming back into town, and on a hunch I said that something must be overheating for the problem not to be present until after an hour of road time. So we pushed it a safe distance off the road, and went and grabbed something to eat. After about an hour we made it back to the truck and success! It cranked back up and we made it the rest of the way home. Since I have driven it to town and back, and to my parents a few times, but never more than thirty or forty miles away. It never hesitates to crank, and sounds ok running. Although I will say, I have not owned a truck before or a V8, but it seems to not have the power it should (my thunderbird with a 3.8L had more punch), and I did noticed both times this happened that that problem seemed more pronounced right before it died. But I don't know if I just feel that way just because I am used to driving cars, and the truck is heavier. When it dies, you hear the starter relay click, and that's it.