System Memory Usage & USB Device Plugged In (MIDI Keyboard)

I have been having problems with my 2007 iMac for the last two weeks. 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 throughout the week, the system started moving slower & eventually unusable because of how long it was taking for actions to occur. 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!

Thanks,

Kevin