I am using the SL-5000D with no problems. I am, however, using SDL-1.2.6cvs. I have found that if I use a different version of SDL (1.2.5 and maybe some others, I can't remember) weird things happen. Sometime rotation doesn't work right, sometimes the screen is all lined. I don't remember multiple keystrokes not working, but mostly because I never made it far enough into a game with another version of SDL.
Perhaps you should try 1.2.6cvs?