Running rough - P0300 OBD code

My Suburban started running rough, the mechanic around the corner told me that the OBD has a P0300 code (random misfire), but is concentrated on #2 cylinder. He said it might be the injector but he's not 100% sure. I changed the spark plugs, cap & rotor, fuel filter but problem still exists.
A month ago, I replaced the thermostat and flushed the system because the temp reached 210 F, then it was fine. Not sure if this has anything to do with it, but anyone has any idea? 54,500 miles.

There are some issues with injectors causing misfires on these vehicles. There is a central injector with 8 poppet type valves. 1 at each cylinder. The poppets are known for getting clogged and can be cleaned (a special tool is used to inject nitrogen into the poppet at high pressure)
I would have a reptuable tech have a look and test your injectors.
If one is found bad, it can be replaced, but I would recommend replacing the entire unit with a updated unit from GM that has an individual injector at each cylinder and eliminates the poppet type.