Not sure what is going on:
1. for some reason sound doesn't change when switching to another patch and hooked up via MIDI. For example, I select sound 1, play something, go to the next patch - however, sound 1 would be till be triggered when playing. Going to patch 3, press keys - again, playing patch 1. Panic doesn't fix it, going to Multi - same thing. Unplug synth all is OK, until MIDI signal is received.
2. When USB is showing, no sound comes out until I unplug USB (this may be a feature???)
This problem (1) started with 4.5, so I went back to 4.0.x - same thing. Never had this problem with 3.x 
Update:
Interface: RME Fireface UFX, latest firmware/drivers
MacOS X 10.6 (latest)
Ableton Live 8.0.2
So, the symptoms can be duplicated as follows:
a. Open Set in Live
b. Ensure Fireface MIDI ports are "on" for sync and track in preferences
c. Start playback in live - "clock sync" is visible on TI display, patch works as expected.
d. If I change a virus patch in a middle of a playback (while live set is playing and "c" is visible on TI2) - that's it. That'll be the only sound until I reboot Virus, even if I change a virus patch while sequencer is NOT running. Looks like once that clock signal is received - that's the only patch I'd be able to play until reboot. Even patch parameters cannot be changed - e.g. ADSR, FX, anything - until I reboot virus. Never seen anything like it before, not sure what is going on...
Help????