Got same result here on my Tandy 1000 SL. Strangely enough, it works OK on my Tandy 1000 TL. I think they should have the same video chip. The TL has the 128K video ram upgrade, so I'm not sure if that has something to do with why it works on the TL but not the SL. Perhaps I'll dig into this further at some future date to be determined.