I got same problem. Been told the ISC packs it in intermitently (?spelling), so it can be good for a while (i.e. when a mechanic is looking at it) then turn to poo. Get it pulled apart n cleaned, dunno how much it'll cost to get someone to do, got told it's sort of easy to do. If that doesn't work, grab a new sensor thing.
Bloody anoying but. Specially when i got holden fans along for a ride, even worse if they're chicks, anti ford sayings spew from their mouth
1st car EA Fairmont Ghia, extractors, exhaust
2nd EF GLi, 'custom' interior, extractors, exhuast
3rd EA Fairmont 5spd, low'd, extractors, exhuast, head
work, 3.45LSD, EL dash, interior+ El ECU
4th EB2 Fairmont, EL dash and interior, BBM, LSD etc etc
5th EB Fairmont Ghia 5L. All leather, lowered, exhaust etc