The problem occours more frequently when there are many particles at many different positions in the level. And it also has to do with the number of stages on top of the main view.
The problem occours very badly in this project: http://files.filefront.com/DemoLevel+V05rar/;10064920;/fileinfo.html especaially if I also activate DoF and blurred Stencilshadows. This version is without source but I could give it to you if needed. Or just the postprocessing part (each effect can be applied through one function call) or something like that.
I already got used to the error, but within thisproject it is just too bad if even the movement seems to lag because of this (the fps stays nearly the same...). And it is hard to create screenshots with each effect visible and without any strange obstacles...