Hey,
you probably have something like an LFO assigned to the TVA of the bass patch...if you're in performance mode, press 'Performance' and 'Patch' together..the patch shows up and you can edit it....here's how to check for an lfo...
Press 'LFO&CTL', and new window appears. On the right there is the column....if there is a value other than 0, the volume of the sound will cutoff and rise when you hold the keys down. Make sure that you do this for all 4 tones (check all four of them by pressing the 1,2,3 and 4 buttons of the 'tone select' row).
Hope this helps.
Nick