I have been having an ongoing issue with poor or intermittent Bluetooth performance with my early 2008 Mac Pro Quad Core 2.83GHZ workstation with factory installed airport and bluetooth radios. Trying to use Apple's wireless keyboard, Magic Mouse, and Magic Trackpad was becoming a frustrating experience. Luckily I found a thread on the Apple's support discussion by others suffering the same condition along with the fix.
Apparently, with the poor placement of the antennaes within the Mac Pro's all metal case, wireless performance suffers causing dropouts and overall poor reception. However, the "fix" is as easy as re-wiring the antennaes.
Here are the instructions. Good Luck.