96 Jimmy Hesitating

I have a 1996 GMC Jimmy is that a mystery. After about driving it for 20 miinutes or more on the interstate, it begins to hesitate/buck. The service engine does not always go on. However, lately it has. I immediately took it to the dealer and P0300 engine code was detected. It was just at the dealer the week before to determine what the problem was and they couldn't find the problem. Even with the service engine light on, they still couldn't tell what the problem was. Had a tune-up done this summer. This is getting frustrating. Anyone had same/similiar problem? Can anyone help?

The P0300 code indicates that the PCM picked up a random misfire.
By looking at scan data, you can narrow down which cylinder the misfire is occurring.