Runs Rough at Speed

[bobgreco]

For the first time since the painting project, we took the truck out for a longer ride at speed, about 10 miles. On the way home, it started to ride very rough around 40 mph and was making kind of a popping or pffft noise from the engine. It also seemed to have a bit less power. Slowed to 35 and it settled down so we made it home OK.

Any thoughts on what this could be?

Thanks for your time!

Bob