[Mike Taylor 3601]

Sounds more like cam walk but should'nt have that issue on windsor due to them having thrust plate between cam and gear,unless it's loose or something else,as cam moves moves back or forth ign.timing changes just like you are twisting the dist,usaully changes with RPM.
I think you mentioned slack between dist/cam gear could be there.
If you are using ford style ign. module may try that,I've seen modules go bad and timing would move around.
It's computer controled may have computer problem?
I don't know if that is any help but might give you ideal.
Mike Taylor3601