I have a recurring problem with cubase 6.03 mac version , it happening from time to time very often just after saving
the file , the song turn out of tune on several vst soft synths . After i close the program and restart it it work again fine for a while and after it starts again to get out of tune .
It is very strange and i cannot figure out where it could came from .
Patrick Mimran
Never heard of this. Are you sure the samplerate isn’t getting switched somehow?
I can’t be of any help other than to say I’ve experienced something similar to this on a number of occasions in the past - seems like an unwanted transpose 
The MIDI note is correct but the note played back is not. Very odd. Restarting Cubase always cleared the problem…until next time …
Aloha and good advice.
Being a MIDI guitarist (lots o’ bender messages constantly
being sent all over the place)
my vstis can on occasion get pulled toward the dark side.
When that happens I shut down and re-start everything ('puter included)
Seems to work.
HTH
[‘-’}
Speaking of which… MIDI/VSTi tuning on my currently open project has just gone completely haywire! 
An MIDI e-piano track is now playing back 3 semis lower than it should be except the part I just recorded, which has seemingly dropped one semitone