I experience this after I switch keyboard group with "ISO_Next_Group"
key, and it is not limited to emacs. So I guess it must be Sawfish's
problem.
I don't know X and I can't say much, but recently Sawfish has a
compilation option which drops support of mouse buttons 6-9. It may
help. (In my case, it doesn't help. The symptom does not seem unique,
but there're some types.)
I found by searching through the mailing list archive that you
suffered from similar problem last year. Sorry that we haven't made
progress in this area, but we're planning the fix the next year.
Regards,
Teika (Teika kazura)