Yamaha Steinberg USB Driver problem

I think I have a problem with the Yamaha Steinberg USB Driver, required to run Cubase AI 5 that came with my Yamaha MOX6. It appears that Cubase loses the Yamaha Steinberg USB ASIO driver as soon as MIDI notes start coming in (a window opens saying the the diver “has been removed”). This behaviour is repeatable and occurs whenever there is VSTi instrument that consumes MIDI events and produces audio (both presumably going through the driver to the MOX).

Here’s my setup:
Acer Travelmate 5742Z, Intel Pentium P6200, 2x 2.13GHz, 4GB RAM
Win7 SP1 x64
Yamaha Steinberg USB Driver V1.7.3 (64bit)

For testing purposes I have also installed the driver on an older notebook running WinXP:
DELL XPS M1212, Intel Core 2 T5500, 2x 1.6GHz, 2GB RAM
WinXP SP3 x86
Yamaha Steinberg USB Driver V1.7.3 (32bit)

Here’s what I found:

  1. Audio test (sound from notebook, no MIDI)
    Acer: When I plug the USB cable in and play a sound through Yamaha Steinberg USB ASIO using a Software Synth, a strange distorted and hacked-apart noise is heard (“krrrk krrrk krrk”). After playing a few notes the sound goes silent, and then comes up OK (sometimes without intervention, sometimes after I reset the audio link).
    DELL: No distortion can be heard, and the sound comes out clean and crisp.

  2. MIDI
    Acer: When I route the soft synth’s audio to the notebook’s speaker, I can play the soft synth using MOX as a MIDI controller.
    DELL: Same

  3. MIDI and Audio
    Acer: After two or three notes no further MIDI events get through. I can still play sounds through the Yamaha Steinberg USB ASIO link. Sometimes when going from test 2 (MIDI only) to test 3 (MIDI and Audio) the driver appears to crash.
    DELL: I can play the soft synth via MIDI without any problems, sound is clean and crisp.

I hope these diagnostic tests (in particular the strange distortions after the USB cable is plugged in) help in identifying the problem. I’d really like to start using Cubase on the more powerful Acer Win7 notebook.

Perhaps another strange thing: when Cubase “loses” the Yamaha Steinberg USB ASIO link, I can click the OK button, and the window goes away (without correcting the fault). However, after that I cannot exit Cubase anymore. It gets stuck (“Not responding”) until I kill it.

Thanks for any suggestions!