Unwanted key repeats while holding down Shift (Mac OS X)

Frequently when I enter text while holding down the Shift key the character repeats three times. I'm using the Mac trial version of the software on OS X 10.5.4. Here's the process:

1. Create an object that requires text entry.
2. Double-click the object to enter text.
3. Hold down shift and press a letter key to get make the first letter a capital.
4. Scratch head in puzzlement as I look at the text "QQQuestion 1" : )

It also happens when I hold down shift to get other characters, e.g. "Question 1:::"