Truck's Not Running Right

[Ole Blue 68]

I drove the truck to work this morning and it wasn't running right. This is the first time I've ever had this problem so any input you can give would be appreciated. It ran fine at lower RPM's but when I started getting up to @40mph or higher it would start to bog a little and then I would hear a little spit out of the exhaust (sounded like the right bank), almost like it was missing. I was about to pull over and check it out when it started to running fine. When I got to work I didn't notice anything out of place.

I was thinking possible vacuum leak but then I ruled that out since it seems to have corrected itself. I thought may be an exhaust leak but I don't think that would cause it to bog. My other theories would be either bad gas in the line or an ignition problem (cap, wire or a plug).

What do you all think?