Low octane tables relation to MAF

[Slammed..]

Aloha gang,car is a 99 auto. I pretty sure my MAF is the problem,but don't know what "triggers" this. At WOT,my PCM will flip to the low octane table,timing anywere from 18* to 21*.I'll reset the PCM (fuses 16 and 23),PCM will go back to the high tables,scan shows 28*.The second scan it will flip to the low tables again. Heres what i did 3 years ago.De-screened,heavily ported the MAF,removed the air flow metal piece in the center and left only the wires exposed.And the table switching has been going on ever since. My o2s read about .880 to .890. I also have gutted cats with o2 sensors,but the problem started after i did the MAF and before cats.Any opinions would be helpful. Mahalo..