I have the same problem, anyone solved it maybe?
I used Cakewalk Sonar, toggled MIDI port "Roland V-Synth XT" on in menu Projects/Options/Sync. I switched Clock Source to USB MIDI on XT in System/MIDI USB.
It can be the problem that I used USB MIDI to communicate with Sonar and I connected at the same time a master keyboard to XT via the normal MIDI port?
If I cannot solve this problem I will buy a Novation USB keyboard for play and use the normal MDI port for clock source from the PC.