Code P0300, engine misfire?

The vehicle listed above has a 5.3-liter LS engine. So when I am driving down the road my car will just shut off but it slowly does it like it's out of gas or something, idk I'm not sure but it's really annoying. I have to put car in N and coast as I am trying to start the truck and sometimes it will not fire up and I'll have to pull over and put in park and wait a couple minutes and it will fire up. I am pretty sure it happens more often and frequently when its hotter outside and it will not let me get on the gas I have to slowly rise my speed up to 60 MPH, I can't go above like 2,500 RPMs or it will bog out and either shut off or I have to let off the gas and its throwing a code for p0300, which I have replaced the plugs and wires just in the last week. I bought the truck from a friend and he was kind of a tweaker and messed things up. I think the exhaust is jb welded and wrapped with the heat wraps and the cats are on the truck, but he has screws in one side in one cat. He said he knocked the cats out of the pipes, not sure if he did or not, I really could use some help to get this truck running right. I just don't want to keep dropping money in places that are not going to help.