You can check the Accounts & Sync in the settings. This will show you what is syncing.
I am going to throw out a guess that you have the Official Twitter client installed and have sync data enabled. I used to get the "lock up" you describe all the time.
The lock up is not really just from the app that is syncing, but it is causing it... Samsung decided when they designed the Galaxy S that the file systems that existed were not good enough for them. So they decided to use a proprietary file system that was developed by Samsung called RFS. This file system is slow, like, Really Fricken Slow. It is the cause of the "Galaxy S Lag".
Even Google decided, with the Nexus S, that they were not going to use RFS on the device, it uses ext4. There are "fixes" out there to convert the RFS to ext4, but they do require you to have root access, a custom recovery (like clockworkmod) and to flash a ROM or a kernel.
Voodoo is one of the best. Roms like Nero3 from TeamWhiskey have voodoo built right in to their ROM.
Now if you don't have root access and don't want to, there is really nothing that can be done to "correct" the lag that you are experiencing... except to disable the syncing of data that cause the lag.