I have the latest version: SX 3.1.1.944.
I really hope you can remember some magical setting

Any ideas about what I could try?
I think I really did waffle too much before, but the real confusion about this is that Cubase appears to be sending and receiving Midi data ok with no timing issues and the keyboard's audio out is responding fine. The problem is how either Cubase OR my Triton is dealing with additional Midi data 'on the fly'....sometimes!