Vanishing sound/output

OK - Cubase 6, Windows 7, 64bit, intrument track and piano sound selected then…

It plays OK for a while then goes barely audible (good signal going in but not coming out) - then it happens on another track. This morning it’s gone pizzicato despite holding key down…

My teacher doesn’t know what this is about and it baffles me…

It happened once or twice with Cubase 4 - did all the usual - restart etc etc…

Has anyone had this problem and know what’s happening?

Is there a problem with your MIDI keyboard? I’ve seen similar when a MIDI keyboard outputs a volume message as the control is broken, and then tracks stay quiet after that.

Months later - we never did get to the bottom of it. I need to talk to the guys at Steinberg properly about it.
I just boost the sound in the mixer to compensate but the volume goes down in every file - not every track though - no automation - Cubase poltergeist…