When I add organ to 4 other MIDI keyboard & drum tracks, the notes of the organ sound drop out & in, unpredictably!

My S90 ES is in Multi-SeqPlay & Utility mode, with Local Ctrl set to OFF.  It’s connected via USB to a Windows Vista machine.  S90 ES Voices are properly set up, and the other keyboard & drum voices sound fine.  All SONAR MIDI track inputs & outputs are set to S90 Channel 1, with each SONAR instrument track set to a different S90 channel (1, thru 5, in this case).

I’ve used lots of MIDI & Audio tracks at one time before on this computer & never had notes dropout like this before.  Oh, and this happens even WITHOUT using the S90’s sustain pedal.

Any possibility that I’m somehow exceeding the S90’s polyphony?

Thanks very much --David