Thanks, everyone for reading through this issue and for all your suggestions. PROBLEM SOLVED!!
Steinberg Support suggested using another MIDI instrument that has a USB port and connect it directly to the computer and test with that and bypass the MIDI interface altogether. Before I got to do that, I compared Logic X on my iMac with my Macbook Pro (since I had it installed on both computers). Logic X worked perfectly on the Macbook Pro. I contacted Apple Support. Did several tests over the phone. No Luck and it was presumed to be a hardware issue with the USB bus in the iMac.
Took a condensed version of my studio to the Apple Store and set up the iMac, interface, sound module, keyboard and reproduced the issue at the Genius Bar. Further testing there proved it to be software, not hardware. Made a painful decision to start over with a clean reinstall of the OS and manually install my programs again and see which one is causing the problem.
After reinstalling my OS, I installed Logic X, then the driver for the MIDI interface. Tested Logic - Problem gone.
Installed Cubase - Tested (with Logic X) problem still gone.
Installed USB MIDI Driver for Sternberg CMC controller. Tesed w/Logic X - PROBLEM REOCCURRED!
Contacted Apple for help removing the driver. They suggested another clean reinstall of the OS.
Sigh
Reinstalled OS. Started over and installed everything so far without the driver for the CMC controller.
Discovered from MOTU website that there was also a newer driver for the MIDI interface. Installed newest driver.
Tested - Perfect!
Checked Steinberg website and found newer driver for CMC.
Installed Tools for CMC v1.1.3. Tested - Perfect!
Installed driver for Audio Interface. Tested - PROBLEM REOCCURRED!!
Contacted MOTU Tech support.
MOTU suggested reinstalling the MIDI driver again.
Reinstalled. Tested - PERFECT!!
The reason it initially worked on the Macbook but not the iMac was that the most current MIDI driver was installed on the Macbook Pro, but the older version was on the iMac. The version numbers were so similar (same digits, transposed) that I missed it (and the folks at Apple). Plus I didn’t have audio and CMC drivers installed on the Macbook.
Lesson Learned: Drivers are a pain!
Case Closed!