I have a 91 pickup that I'm currently forced to limp cross country. My truck was running fine until the other month I put premium gas in it and it started to occasionally sputter. I'm having to drive it from Colorado to Alabama now I got close to the Kansas border and it started sputtering hard. I went from cruising at 75 to putt putting at 50. I stopped at a Napa and swapped out the spark plugs got back on the road and was cruising good. After getting in Kansas stopped at a Arby's and after I left it got maybe 10 miles down the road and it started sputtering again. The only difference from before Arby's is it got dark so my lights where on. I turned my A/C and it seemed to clear up. I didn't find a vacuum leak. I'm really lost

 

What engine?

Is your check engine light on? can you pull the codes?

 

When was the last time the fuel filter was changed?
Putting premium gas in it won't be the cause of this. That's just coincidence.