Not able to go past 30 every once and a while

Not able to go past 30 every once and a while I have a 1995 k2500 suburban so I'm not sure this applies to this category but it was the closest, but like the title , every once in awhile I can't go past 30 its like being stuck in 2nd/3rd gear, I had a mechanic look at but he said I'd be better off with a new truck, so any help would be much appreciated!