This is the only info I could find about a code 90 anywhere. Its not even listed on most trouble code lists.
SERIES FUEL/AIR INJECTION CODES ON VEHICLES WITH DUAL OXYGEN SENSORS REFER TO THE LEFT OR FRONT SENSOR. EXCEPT: 1984-1988 3.8L ENGINES: RIGHT SENSOR
How did you come to the conclusion that your MAF is in working condition? Every time I see a code 157 on this board, it turns out to be a bad sensor. The MAF is one of the hardest to test accurately, and often the only way to know for sure is a CEL. I would recommend you replace it unless you already have.
Code 998 basically means it is in limp mode.