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:

SyncServer Problem Post on Mac Forums

OS X 10.5, SyncServer takes 100% of CPU
This article suggested resetting Sync Services, following the instructions in this Apple Technote:

Open Terminal and paste in the following command, then press Return:

/System/Library/Frameworks/SyncServices.framework/Versions/A/Resources/resetsync.pl full