Re: S4 conversion help
The chap in Hull is/was called David Ellison. We bought lots of cars from him during "the S4 years". He used to get them from places in Europe and then convert them and sell them to Alfa specialists.
The Seaking conversion was the official Alfa UK conversion and cost quite a lot of money from memory. Seaking conversions are identified by a small plaque in the drivers door shut. I do not know who did the Lombarda conversions, unfortunatly the chap who owned Lombarda when your Spider was new passed away a few years ago.
Have you tried the obvious wheel balancing?! If they are in balance you might find a wheel rim is out of shape.