I have a feeling its your boost controller (t-peice)
Ok so i tested my friends turbosmart single stage controller. On lowest setting it was still at 0.6 bar-0.65 bar- spiked to 0.8 bar at times..
im like, hmm lowest setting and its still more than stock boost (0.5 bar)?? *confused*
But sorta didnt bother me for a while...
Anyways, put my old gfb bleed valve back on, and i can seem to turn it right down and be stable.. I mean, it will still spike at times under full load, but that would be normal since we do have bigger exhaust, pod filter etc.. (since more air is still coming thru, its pushing more in)
Thats the way i think of it.
Maybe try another bleed valve and see if it helps in any way..
Ps- change all your spark plugs again.. Then check after a week and see if that plug is still going white.. If so, take it to a shop n see what it could be..