Your HC and CO are both elevated so you are not running lean. I plugged your numbers into a Lambda calculator
http://www.smogstats.com/Lambda_CAT.aspx
and your air/fuel ratio reading is .997 which is slightly rich. This should cool the burn and reduce NOx. According to the calculations, your CAT efficiency is 57.65%, not real good. Looks like the CAT could be getting weak, but the CAT is the last thing to condemn.
Couple of questions:
Does an 88 have an EGR valve? What does a Steve Wong chip do to the engine tuning?