See if the fuel system cleaner helps, that's probably what's causing most of the problem.

Sometimes the o2 sensor can just get worn out (like, the voltage it reads is no longer accurate) but not to the point of throwing a code. That can make you run rich but only during the time the ECU is in open loop mode. You can't really clean it, just replace it.

But yeah, my guess is it's something inside the fuel system. Possibilities include a worn fuel pressure regulator that's running a bit higher than normal. Also, any number of spots could be gunked up causing fuel starvation and knock, which causes the computer then dumps more fuel to get rid of.