I too am having problems with VB6 SP6, not with SQL Server, just with VB6 in general.  Seems to be a little more unstable than it usually is.

I am only running MDAC 2.7 and not 2.8.  So maybe it has something to do with VB SP6?

I have searched numerous SQL Server websites and everything in microsofts web site, but have not been able to find a solution to the SQL Diagram issue.