I am using DM v4.3 and having the same issues as SharonEve. I have not yet resorted to contacting tech support at my carrier, Verizon, because I've suspected it to be a bug. Online reading seems to indicate this is so. The only successful way that I have found is that mentioned above, to deleted the calendar db prior to a hardline sync.
I've seen mention of a calendar patch here on Blackberry Support.
However, when I finally gave in and opened IE (being a staunch FF user), the site application launcher indicated that it would not interface with my phone. This patch seems geared at the 8330 and 8830 only. My poor little 8130 is left behind! How To reset the Blackberry device calendar
Best wishes, I'll hope they come up with a patch soon!