Parallels Desktop v3.0 corrupts downloads on Mac Pro but not on MacBook Pro

Parallels Desktop for Mac version 3.0 is corrupting downloads and updates on my Mac Pro, but not on my MacBook Pro. Symptoms include the message "The digital signature for this package is incorrect. The package may have been tampered with or corrupted since being signed by “Apple” when downloading a software update. Downloading a dmg software package fails at the end of the download with a "Verification failed" message. Trying to mount the package manually fails with codec overrun error. These are symptoms of a corrupted download.
The problem disappears when I uninstall Parallels. It reappears if I reinstall Parallels. My Mac Pro and MacBook Pro are nearly identical in installed software. I have reviewed the problem with AppleCare in excruciating detail (an hour yesterday and another hour today with an AppleCare Specialist). It is a new issue to them. We have send a support request email to Parallels.com.
This has been a difficult problem to troubleshoot. I had tried every suggestion that I could find on the forum under "Digital Signature is incorrect", "Verification failed", "codec overrun", etc., without success. I won't go into the painful details of the troubleshooting process, unless encouraged. But I will update the forum when I know more. Meanwhile, if you have any ideas please pass them on.