iMac Problem - System Memory Usage & USB MIDI Keyboard

I thought I would pick your brains on a problem I've been having.  Was curious if any one else has had this problem or might have a solution.  It has definitely put a damper to my writing lately!

I'm using a 2007 iMac, 4 GB of RAM installed, Lion OS.  First, it began with the ";White Screen of Death.";  After a trip to the Apple Store & a fresh erase & reinstall of the system, everything was working fine...for 3 days!  While reinstalling programs, the system started moving slower & eventually unusable because of how long it was taking for actions to occur.  Problem started with NI Service Center, but don't think that's the culprit.  Tried various ideas from searching the net, but to no avail.  Thought it might be my hard drive failing so I decided to go back to the Apple Store yesterday.  While there, the problem could not be replicated.  Everything worked fine & he said that everything looked great.  No major problems, tests all passed.  Got home, plugged everything in, fired it up & same problem as before!

I checked Activity Monitor today.  The biggies on CPU activity are ";kernel_task"; & ";SystemUIServer";.  When viewing the System Memory, I noticed something interesting.  The system memory was almost completely used up which is 4GB.  Hard Drive usage was fine, 250 GB free.  

Once I unplugged all USB devices & rebooted, I discovered that everything was working just fine.  I usually have a USB keyboard, USB MIDI Keyboard, and Firewall Maxtor External HD plugged in.  I have narrowed it down to the Korg USB MIDI Keyboard.  Without it plugged in, there was around 3.13 GB of free System Memory.  When I plugged it in, it immediately started dropping and ended at around 7 MB of free memory!!!  I've tried different cables and that did not resolve the issue.  

I would appreciate any ideas or suggestions!  I am completely stumped as to what the problem is.  I live 2 hours away from Apple Store...a 3rd trip is not possible this week!  

You might want to see if there are any driver updates for your Korg keyboard. If indeed that's the culprit, that'd be where I'd start. They should have them available for free on the Korg website.
Thanks for the reply Jim.  Little more research on the problem.  It also happens when iPhone is plugged in to computer.  Someone said it sounds like a ";bus-powered USB"; problem.  Both devices were not using external power source & plugged in to USB for power.  No solution yet though...
Interesting. I've heard of this sort of thing before, though I don't think it's typical. This may be a stretch, but in the interest of troubleshooting, you might also check to see if a particular USB port is the culprit. This will probably be helpful info if you're having the machine diagnosed for repair.
I've had similar issues with too many security dongles, external drives, and other devices on a single USB hub. I can run my iphone via a hub but my ipad needs to be plugged into the computer for charging purposes. As far as the keyboard goes, if it's usb compliant it shouldn't need drivers for the Mac. I'm currently trouble shooting an external drive that makes restarting an issue and I see similar results, white screen, long or stalled start up, etc. Things work great when it's not plugin.
Turned out to be my hard drive failing.  Got a new one in there & everything is working perfectly again.  Plus tons more space for all my VDL compositions & arrangements...  :)

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