can be hard to correct cos there can be many possible problems, and sometimes lots of minor ones add up to something more significant.
*spacers: might be contributing if they are not machined perfectly and don't seat the wheels properly
*spiggot: does the centre hole of the wheels actually sit 'over and onto' a matching-diameter spiggot on the spacers? a standard alfa wheel has a 58.1mm spiggot hole; much of the force between the wheel and hub passes through the spiggot rather than the actual bolts, so this interface is important; some aftermarket wheels and spacers do not provide the same diameter spiggot, so plastic rings are required (eg: wheel might have 70mm spiggot hole and spacer might have 50mm spiggot, so a 5mm wide ring is required to bridge the gap)
*PCD: is the bolt pattern on your wheels suited to the spacers/hubs? as opposed to using 'wobble bolts'; for example the standard alfa PCD bolt pattern is 5x98mm but some people fit 5x100mm wheels and use 'wobble bolts' (that have slop in them) to accommodate this; however, they are prone to not fitting perfectly
*suspension bushings: any wear?
*suspension arms: any of them bent?
*bearings: unusually worn out?
*dampers: worn out?
what size are the spacers and are they essential? i figure so.