Not sure about this one....

So I did my headers awhile back and had strange idling vibrations after...turned out to be a cracked plug on drivers side closest to firewall...car ran GREAT after that with all my mods so far especially with the nice headers...

Now recently I got some CELs of O2 sensor i think running lean and still ran fine then awhile later on way back to school light came on and checked it on the fly because my car started what felt like missing and losing power while moving up grades on highway or putting in more throttle...code said drivers side precat O2 showing little or no voltage...just weird that now that im back at school code went out on its own after a few short driving trips...
then yesterday I felt the same missing feel/losing power after driving around for more then a short trip (probably 20mins or so)

Is this still just bad O2 sensor drivers side precat like code says,since it only really gets a noticable drivability issue on trips longer then around 15-20mins?
maybe the precat O2 got messed up when I had drove a few days when i was diagnosing the bad sparkplug and since it wasnt really firing it was reaaal rich and "fouled" out the O2 so to speak but only really gets bad once the car is been running for ger then 15mins?

any thoughts would be greatly appreciated as this is frustrating that the car runs awesome but after awhile on the road it starts messing up until it throws that little or no voltage code which case i assume it goes into LIMP mode and then the problem goes away, altho im sure my gas mileage cant be good with that light on