2004 GMC Engine Problems

2004 GMC Sierra 1500, 4.3 V6
Problem history.
I am fairly certain that I received bad gas 2 weeks ago.

First indication was the engine being slow / hard to start in the morning. Normally the engine fires immediately, this time it cranked for 10 – 15 seconds then fired but was running rough and sounds of back firing.

Engine trouble codes appeared: Random multiple cylinder misfires.

Removed and replaced fuel filter and used “dry gas” – no help.

Engine running rough at idle, lower power at speed, and sounds of back firing.

Reset codes. They reappeared: Misfire #1 Cylinder, Side 1 number 1 O2 sensor voltage high. Left bank running rich.

Removed fuel from tank, changed fuel filter again, put in fresh gas and bottle of dry gas.
Still running rough.

Plugs, cap and wires were changed approximately 35K miles ago.

Changed coil. No Help

Changed crank position sensor, (did not take to dealer to put in “learn mode”) – No help

Checked spark plugs; #1 was fouled – changed all 6 plugs.

Changed Side 1 (left side) #1 (before cat) O2 sensor. It was fouled.

Reset codes.

Codes reappeared: Random multiple misfires.

Engine feels to be running rich; exhaust smell seems to confirm this. Runs rough at low RPM, some better at speed.

Any suggestions on the cause for engine running rich / misfiring?