Manufacturing defect in Expert Mouse Pro

"I am about to receive my third unit, all for the same problem. Kensington tech support told me that there is a defect with the optical sensor. The problem's symptoms are consistent: moving the cursor down using the ball results in hesitated movements. Moving the cursor up or side-to-side is never a problem, accurate and smooth as glass. I was also told that replacement units are on the way, although it may be a few more weeks before they are available."