MIDI Patch Change Causes Dropout

Using Cakewalk's built-in soft synths - why would chaning the patch cause a drop-out?
The buffer size needs to be small in order to avoid unnatural latency
 
Keyboard controller to MOTU Fasttrack USB MIDI adapter
 
i7 CPU 860 @ 2.80 Ghz
Win7 64
SONAR X1a 32-bit
Advanced System Settings: Performance