Looks like a misfire to me.... It will spike lean with each misfire since raw oxygen (and fuel) will be going into the exhaust. That'd explain the lack of power, blinking CEL, and misfire counts.
No misfire codes present at all? That's unusual given the fact that the CEL was blinking, unless it went away for awhile during driving. If it starts blinking again, pull the codes. I bet you'll see a misfire code setting. Which sensor is reading this? It'll help isolate it to a particular bank.
Going to 20% is normal during decel when coasting in gear since you'll be in a nonfueling state. Essentially, you are reading atmospheric oxygen content (20.8%) at that time. Many systems self calibrate during that driving state to establish a free air reference. However, if the OP was under load when this was occurring it is almost definitely a misfire.
2011 GT 6MT (Cookie Monster) - The 1/4 life crisis. Brembos, 401A, drop top, Grabber Blue.
1974 Dodge Challenger - The money pit. 408 SB stroker, Megasquirt 2 EFI, 904TF, 3.91 gears. 16MPG and 520HP goodness
2004 Hyundai Tiburon - The winter beater.