I had this exact same problem. I left it alone, but it got to where 3 of the cylinders were misfiring in one trip around town. I replaced all of the spark plugs and wires, since I had recently gotten the truck from someone else and didn't know how old the wires were, and that fixed the problem to where it only happened about once a month.
Then it got worse. Cylinders started misfiring again and some days my truck just wouldn't start at all. In the end, we had to tow the truck home and replace the fuel filter and pump. I haven't had a misfire since then, and that was about 4-6 months ago.
Start with the spark plug wires, then if it persists, replace the fuel pump (be sure to test the current first, though, because fuel pumps are expensive).

Also, not sure what you mean by "mechanics said wires, plugs - new…", but if you already have new plugs and wires check the fuel system.