1995 jimmy misfiring and gas buring real rich

Hello, i have a 1995 jimmy slt 4x4, the truck is running really rough and gas burning real rich.(only getting 200 miles out of tank of gas) i just purchased the truck about 1 month ago. when got truck the check engine light was on, but ran pretty desent. well i took to AZ and had them scan for codes.it came back saying bad 02 sensor and random misfiring. also had to replace the cam shaft sensor. i replaced the o2 sensor and installed new plugs, wires, cap, rotor, and new MAF. the 02 code went away but the misfriing codes is still there. ( code p0300). i had the coil and the ignition module checked and tested fine. i rechecked the gap on plugs to verify set @ .045 and they are. Last week light went away for 4 days , but now is back on. the truck has ran rough and rich every since replaced all parts. Can anyone please tell me what i need to replace to fix the problem. Thanks for any help !! Mike

how many miles??pull the spark plugs & see if any are wet(not firing)
run a compreesion test.

Given the history of the CPI engines, I'd lean toward fuel leakage in the plenum either from the regulator and/or the fuel lines aka "Nut Kit". A fuel pressure leak down test is a good place to start. If you don't have a fuel pressure tester you can do a visual inspection by removing the vortec cover and IMTV (easiest, less invasive) , or pulling the upper plenum (much better view) and looking for washing. Washing is recognized as golden spots that are very clean as a result of the leaking fuel washing away carbon deposits from the plenum which should otherwise be black.

Fuel leakage in the plenum can cause a strong fuel smell, misfires, plug the EGR with carbon chunks, hard starts, stalling, and even engine damage due to the excess fuel leaking into the engine oil.