1996 4Runner intermittent rough ride.

Hi, I have a 1996 toyota 4runner sr5 (6-cyld.), Two-wheel drive model, with 76k miles. I had a full tune-up at 60k including changing the spark plugs and wires, but that was nearly 4 years ago (i don't drive that much). Last month I started having a problem where the car would occasioanlly shake quite violently on a cold start and continue to do so for several miles of driving. It feels like it is going to sputter and stall out, but never does. Once the shaking passes the car runs as normal and the shaking does not return. This has happened to me twice now. The second time it occured caused the "check indicator" light to turn on. I promptly took the car to autozone for a free OBDII scan and the error codes they pulled were P0300, P0302, and P0306, which are listed as cylinder misfires (cylinders 2 and 6). Because my problem is intermittent I am inclined to think it is either a fuel injector that is partially clogged, or a bad electrical connection, such as a loose spark plug wire, or loose coil pack, but any advice would be greatly appreciated. I've added some marvel mystery oil to the gas on my last tank, but I think I'll get some proper fuel injector cleaner, perhaps sea foam?