this may not help but here goes anyway
i had very similar problems with my now deceased vectra and had to spend many a weekend taking the idle control valve off and giving it a real good clean with carb cleaner
problems were very erratic idle anything form 0 - 3000 rpm mainly when engine warmed up sometimes would start and stall when cold when driving felt like accelerator was sticking changing gear a lottery as to wether it would rev high or stall
i found that leaving it soaking in carb cleaner (block holes) for 10-20 mins then scrape with small screwdriver was best way to clean it
one on vectra came apart in 2 sections and had rubber washer which if not seated let air in same as gasket on housing
main cause for carbon build up was the resurculation of exhaust gasses and oil fumes from rocker cover/breather pipes gave all these a good clean and ran fine until the good lady wrote it off
sorry its a bit of an essay but hopefully it may be of use
ps vauxhall made a mod to the breather system on later models to try and cure the problem
colin