Xvfb does not release memory

We want to create screenshots of websites for our software, and we are doing this by using CutyCapt with Xvfb. Unfortunatly every time CutyCapt takes a screenshot, the memory of Xvfb increases .. until the memory of the boy explodes. This is not bound to CutyCapt, it seems that the Xvfb doesn't clean some memory when the application quits. Are there any solutions (except restarting Xvfb (which we do now via cron as a hotfix)). I start Xvfb with Xvfb :1 -screen 0 1280x1024x24 (in a script of course ;) )