Date & Time in Menu Bar disappearing

I am using a Mac Pro and I have Parallels 5.0 installed running Windows 7 in coherence mode. The last couple of days, after I have the machine running for a while, everything on the right side of the menu bar is disappearing, although if you hover over the items where they should be and click, the menus for the particular items come up, such as volume, eject, etc. I have had to completely restart the system to get them back. I tried switching from coherence mode to window mode and visa versa to see if it was Parallels messing with things, but that didn't do it. I'm not sure what is going on. Anybody seen this before?

Thanks