It could be anything mate.

- old cables / connections not optimal anymore
- it could be the settings in cubase, or in your soundcard drivers

Did you tried to measure another imput into your sound interface with your esx?
Did you checked the stereo settings on your midi channel if they have the same routings?



I'm not really known now with cubase to give you a solution. The only thing I remembered as soon as Ableton with Midi was available I've never used cubase again.