I would suspect the EHA valve is malfunctioning. The EHA is also supposed to correct for altitude differences, might be worth a check. EHA valve doesn't need to be 16v specific, the valve from an 87 2.3 8v is the same. Might be worth snagging from the junkyard just for the sake of troubleshooting. Usually you can test the EHA amperage with a multimeter but since your problem is erratic it may not appear out of order. Look for part number 000 070 39 62. Hell, I might even have one I can send your way.