Sounds like the metering unit (probably the EMFPA) is goosed, drifting out of calibration, perfectly normal this far into it's life.... very dangerous as it's going lean.

Also just a side note, the fueling should be set using a wideband lambda at full load WOT not a CO meter a idle, setting the idle CO does not guarantee it's fueling correctly on boost, by doing that you are assuming that the whole system is working correctly which is a big risk with these 25 year old fueling systems!

Replace the EMFPA (or whole metering unit for a known good one if you can) and get it setup properly.