code 300- multiple misfire

I have a 2005 Yukon XLT 1500
I have been getting code 300- multiple misfires for some time and sometimes I get misfires showing on #7 only

Symptoms:
Running rough on idle and when driving
smelly exhaust
Black tailpipe

I did the following with no change:
1. Changed plugs - Bank 1(drivers side) were all sooty, bank 2 were fine) - no change
2. Changed O2 sensor front of Cad on drivers side-no change
3. Cleaned back O2 sensor- was black
No change
4. Swapped wires on #7 and #5 - no change
5. Swapped Coils on 5 and 7 - no change
6. Swapped injectors on 5 and 7 - no change (although I may try tht again because I may have swapped 3 and 5 by mistake)

I recently received a wireless bluetooth ELM327 ODBII reader which shows graphs on my smart phone.
I added the 4 O2 sensors and can see all the sensors switching except for drivers side back of CAD mostly flatline at 0. Jumps a bit once in a while. All the other sensors change almost constantly.

Question:
Is it likely just the o2 sensor or could it be a plugged CAD or something else?

Strange (hopefully cooincidence) but this all started a year ago just after changing the fuel pump.