This is a guess but it could be a thermal problem in the audio power amp section. The strong sounding voices require lots of power output to push the sound to the speakers. This in turn triggers a heat problem in the audio out circuit causing the change in sound. One test is to put on headphones and disable the speakers or other means such as use the line out jacks to your stereo system. What happens if you switch to another saxophone voice while playing a note? If the sound recovers to normal and goes bad again, it isn't thermal. In that case it would seem to be a problem with the sample playback somewhere in the computer circuits. If you do find out the problem I would be interested in knowing the outcome.