1996 Blazer P0300 Code

I have a 1996 blazer 4.3L with about 120000 mile on it. I recently noticed the check engine light came on and the vehicle runs rough. I checked the code, p0300. No other codes. I cleared the code, it came back after about 3 minutes of running, when the engine started running rough and backfiring out of the exhaust. I changed the plugs, wires, cap, and rotor about 2 years ago. When this problem started, I changed the plugs, cap, and rotor. No results. I changed one of the 02 sensors(both 02 sensors before the catalytic convertor are now new). I tried fuel injector cleaner, didn't change anything. ****vehicle has a new fuel pump and fuel filter, good fuel pressure.
Anyone have any idea, I would really appreaciate the help, I don't know what else to try and I am sick of just throwing money away.

See link:
http://www.obd-codes.com/trouble_codes/generic/p0300-random-multiple-cylinder-misfire-detected.php
My 99 had the SES light start flashing as I was driving and that was due to the P0300 detecting multiple misfires. Looks like it was leaking 4wd solenoid causing a vac leak leaning it out only at high speed under heavy loads. Could be a bunch of things so read the info and good luck - if so inclined get the dealer to scan it and they should be able to narrow it down and not throw parst at it, but...