On an underpowered G5 iMac running OS X 10.4, I spent a lot of time troubleshooting performance issues. One of them was the "spinning beachball" effect, where some process would take over for a moment, resulting in everything else hanging until it released some CPU.
On at least one of these instances, launching Activity Monitor showed that Sync Server was taking up 100% of the CPU every few minutes.
Apparently this is a common problem on both 10.4 and 10.5:
Open Terminal and paste in the following command, then press Return:/System/Library/Frameworks/SyncServices.framework/Versions/A/Resources/resetsync.pl full