MIDI problem: M-Audio Keystation 49e "freezes"

I use my M-Audio Keystation 49e extensively for music projects in GarageBand. Around the start of this year, I started suddenly having problems with it.

After a brief amount of playing or recording with the keyboard, it freezes up. GarageBand stops reading MIDI data from it. It becomes entirely unresponsive, with GarageBand not registering any more MIDI signals from the keyboard. This light that blinks in GarageBand to indicate a MIDI signal was received no longer blinks, and GB also doesn't acknowledge if I change the volume on the Keystation.

If I had any keys held down at the moment of the "freeze", those notes keep playing as one long note even after I release the keys. This happens whether I'm recording or not. When recording, the notes show up as a continuous line, confirming that GarageBand really does think I'm still holding those keys down.

The only way I have ever been able to get it to work again is to switch the Keystation off and back on (or unplug the USB and plug it back in, which is of course the same as switching it off an on).

The freeze does not seem to happen at any predictable point. Sometimes I can play for 15 or 20 second before it freezes and becomes unresponsive, other times I'm lucky to get two notes in before it freezes, and sometimes it freezes before I can even hit a single key. It's become a huge problem, of course, as I'm lucky to record even a couple bars at a time.

The problem first started while I was still using OS X 10.5. I hoped that upgrading to Snow Leopard 10.6 might help, but it didn't change it. I hoped that upgrading from iLife '09 to iLife '11 would fix it, but that also did not change it.

I just did some testing using SimpleSynth instead of GarageBand to see whether the problem is limited to GarageBand, but the freezes also happen in SimpleSynth, regardless of whether I'm using the Apple DLS Sound Set or a sound font as the audio source.

Next I tried swapping the USB cable, but the freezes continue with a new cable.

Is it necessarily a hardware issue? I'm doubtful that the problem is with the Keystation itself. I'm suspicious (just a strong gut-feeling) that the problem is with my MIDI setup on the Mac itself, but I'm not confident about how to go about investigating it or fixing it.

Anybody recognize this problem? Or does anybody have an idea what it might be, and what I can try to see about fixing it?