Stock '94 M6, cags unhooked. 55K.

About a month ago it started stumbling, running rough/unevenly, noticible
with light accel in 6th. Got worse till it felt sluggish in all gears, mileage
26+ on highway though. Changed plugs and wires, (still original), wires didn't look too bad, except coil wire had some corrosion at coil post (it's been replaced and the coil post polished) and ohms weren't too high, plugs were sooty though, both banks, couldn't tell which side a plug came from by looking at it.

Test drive was great from the start for about 7/8 minutes then it started
the roughness again worse than ever. Could barely cruise at 60 in 6th, went down to 4th and headed home.

Read some posts and decided to unhook both O2's and try it. Started out
right where it left off but then the more I drove the better it felt. After
10/15 minutes there was a slight roughness in 6th but it was running as good as it had in the last month.

Checked O2's and the P side was a little sooty but the Dr side was heavy soot
and the "vents" were almost plugged off. Drivers O2 was replaced by dealer
about 5 years ago.

Going to change sensors now but wondering if the soot in the drivers side pipe will dislodge and screw up my new O2?

What would cause it to run so rich in the 1st place (if it is)? Btw the original plugs, delco 41-906's, had no protrusion on the neg electrode just the pos (center) one. All 8 the same and with .064/.065 gap. Could that have caused a weak spark which left the soot?

Thanks for any advice.