fixed. put 300 miles on the odo since the fix... i'm almost 100% sure it was the 2 rear O2 sensors not being tightened all the way down OR the header to HFC not being tightened down first before i tightened everything else down.. with this, i followed the TS guide on the codes and cleaned my air filters and re-oiled them, sprayed my MAF sensors clean, and even ran Techron+ through the tank to clean out the injectors.. whatever the cause was, it's gone now.
lots of "omg help" threads out there on this code with NO solution, so if anyone knows anybody having this issue, link them my legacy thread.... i couldve avoided all of the headache if i followed these simple steps:
when installing HFC:
1. install O2 sensors with anti-foulers as tightly as you can without breaking anything
2. tighten additional bung holes (either the block off bolt or your AFR sensor)
3. position HFC onto header with gasket
4. tighten down the 3 bolts for HFC-header as tightly as you can (ie 50ftlb~, aka as tight as you can since you cant fit a torque wrench down there) to get proper seating of gasket
5. once your HFC is mounted, install your cat bracket, then your other piping, make sure you line everything up to the best of your ability. i used a channel lock to press the HFC and cat bracket together to give the O2 sensor more room, by tightening the bolt and pressing them together, the HFC moves away from the transmission (this may not be necessary if you have a J-shape anti-fouler or you run a manual trans)
6. once all piping is installed, give the O2 sensor and anti-fouler another tug to assure proper sealing
note: for people who have never used an anti-fouler (like me), you need to seal the O2 sensor against it (washers must seat against anti-fouler to seal). with the ones provided by Z1, they milled the inside of the anti-fouler just a little too small, so the O2 sensor fits in there VERY snug and makes a TON of squealing as you're tightening it down. no worries! just crank down until you properly seat the O2 sensor against the anti-fouler, if you don't, you'll catch a code.
if anybody needs any help or tips or photos, lmk and i'll hop under the car and snap a few photos.
lastly: before i started snooping around for the issue, i was getting the code every 40-80 miles. VERY ANNOYING.