Hello Bernie
A few things to try
1. Check the SD2 to see how many voices it can play simultaneously and also how many Midi channels it can receive simultaneously. (This may be a limiting factor)
2. Make sure when you send information it is always on separate channels, such as lead = 1, accompaniment = 7, 8, 9 etc. (Whichever board/computer gets there first usually takes priority.
3. Mixing various external Midi boxes can also cause problems, so keep them to a minimum. (Also make sure they are the type you think they are, as most of them look similar)
Hope this helps

Bill