I had this problem before. I got a pioneer A0whever dvd burner (top of the line). whenever i burn a dvd, it only burns at one speed. I always have problems (error messages) aswell. It was soved for a short time, i came here and told my problem, someone gave me some links to plug-ins that i could install. Totally forget what they were called though.
You said it could only burn at one Speed. What speed is it? What media are you using? You may want to research into which Media is best for your Scenario, which I am assuming is, Your Burner only burns at a fast speed. 8x or 16x maybe. Do any of that make sence? JBurden