transmission freezes when downloading large torrents

after starting the download of a large torrent, transmission works with 100% CPU and freezes for some seconds, sometimes also 20-30 seconds. After that it apparently does not have problems anymore

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: transmission-gtk 2.77-0ubuntu1
ProcVersionSignature: Ubuntu 3.8.0-19.29-generic 3.8.8
Uname: Linux 3.8.0-19-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Mon Apr 29 14:28:18 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2012-10-29 (181 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
MarkForUpload: True
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: transmission
UpgradeStatus: Upgraded to raring on 2013-04-25 (3 days ago)