I had problem with the synth randomly processing only part of the midi voices. (windows XP AMD 64)
The solution was:
1. Download process explorer from www.sysinternals.com
2. Install / open
3. Right click the Vsc32.exe icon in the process explorer folder tree.
4. Open set priority menu
5. Right click the real time 24 option

You have to do this every time you start the computer.