Midi track drops out and loud buzz - have to re-start Sonar - maybe linked to freeze synth

Strange problem just started happening on my DAW setup. Regular project (around half a dozen midi tracks) and looking to add some strings via DimPro. Using an Oxygen 49 midi USB controller as the input device and after a few notes the track just drops out and I get a really loud buzzing that I can't kill other than by exiting the program.  The soundcard is a Firewire Saffire LE running under ASIO.  All the latest patches and upgrades have been installed and it's a Quad Core 8Gb RAM Win 7 64 bit box with oodles of disk space.  This doesn't happen on any other soft syths (some of which are also DimPro sounds) but it looks like it might be linked to the fact that I have frozen the synths on tracks already completed as this makes the project loading time significantly faster.  When I unfreeze the synths - no dropout.  Any ideas welcome as this one has me beat.
 
Thanks
 
Andy