You just described exactly what my old '75 Chevy Vega did a few months back. Turned out that the fuel pump was bad, it would run for a while until it got so hot that the internals would expand and the pump would sieze. Let it sit for about 15 minutes, and it would fire right up and go for another hour or so. On the highway I wouldn't be able to get over 55-60mph as the fuel pump couldn't keep up with the demands of how much fuel the engine needed. Check out the fuel pump, and possibly alternator.seasaw said:going down the road at 65 mph.then dropped to 55 then 45 hen 35 then i pulled over.noticed that the voltage dropped shut it off to see what was going on.then got back in to start it then nothing happened wouldnt even crank. let it set a second then it started.started to go again and could not go over 35.shut it off when i stoped to have breakfast got in started right up and run fine. ran a few places then could not get over 35 again.gave it gas then it acted like it wanted to die.it idles fine.for a few minutes it runs good then goes nuts rpm is irratic along with the voltage.