SR running weak (O2 sensor, knock sensor?)

[Jsquared]

Alright, this started intermittently about 3 weeks ago and has gotten more consistent in the past couple of days. When the car is in a "bad mood" it just isn't making the same power... as if the knock sensor is retarding the timing way back, but that doesn't make sense since i'm only running ~11psi on 93-octane (Walbro pump, FMIC, dyno-tuned S-AFC). same symptoms though... makes boost fine, no leaks that i can find, doesn't pull like it normally does, exhaust sounds a little funny (kinda "gargly" sound for lack of better description) and off-throttle it pops a little as if it was running way rich. i thought it might be the oxygen sensor (since my stupid self didn't replace it when i swapped ~ 6 weeks ago) giving up the ghost, so today when it was in a bad mood i pulled over and unplugged the O2 sensor plug. Ran kinda the same, maybe a hair worse, but the same kinda crap it has been doing recently. Plugging it back in didn't really change anything. I thought this pretty much narrows it down to the oxygen sensor crapping out, I just wanted to get your guys' opinions.

BTW, '87 300ZX turbo, 12mm thread diameter, right?