I've enabled 'link-level flow control' on my Cisco switches but it does not resolve this problem.
I opened a SR about this issue : SR 3-5713617231
I'll keep you posted about what Oracle have to say on this problem.
I don't think this is related to network performance but similar problems have been reported again and again. Have you tried enabling the xkb extension or setting the repeat rate for Xservers globally as suggested in these threads?
I tried to "play" with autorepeat options for XServer with no luck.
I also try to add the XServer autorepeat options to XNewt but it doesn't seem that XNewt supports -ar options.
I also bypass VDI route to log into Solaris 10 Default Desktop (JDS) instead of VDI assigned desktop and I've still got the issue.
Changing Keyboard Autorepeat parameters directly into JDS don't resolve the problem either.
By the way, when using OVDC 3.1 the issue disappears. So it seems to be a DTU problem.
FYI, the SR opened about that case is still under investigation by Oracle as they can't reproduce this problem in lab.
Just a thought, in between the environments, are you running the same firmware on the DTU's your testing from? We've seen this happen before but it almost always fixed itself so no one ever complained..