I had a similar problem with the SZR660 (an xtz660 in drag) after i kinked the vac hose under the tank. The problem manifests itself as a misfire which goes away at lower revs. It's caused by fuel starvation, casued by the fuel pump not working properly.
A split or poorly attached vac hose would manifest similar symptoms and would probably get progressively worse.
1987 XT600 1VJ
"Why do it? Because we can, and we want to." Guiseppe Benelli