Bracuru:
Many thanks for your advices. Unfortunately both did not help
.
Additionally I emptied the temporary files folder. Rendered the 35mins video again. Did not help at all. Once the project is re-opened I am able to move the ruler less than 10 times and it stacks at one clip and dosen't want to move at all.
I noticed as well that my procesor works like crazy once I open this project. It does not act the same with other projects. What I did wrong?!
Madness. I am close to cry or to kill my lap. It is so frustrating to realize that 40-50 hours were just wasted in front of a computer.
I noticed as well that my procesor works like crazy once I open this project. It does not act the same with other projects
There's probably somethibg specific about this particular Project which your Your CPU can't handle. However you say;
The video last 35mins and consists of 250 clips and all of them were rendered.
Background Rendered?
In any event, 250 Scenes in a 35 minute Clip is too many, IMO. Combining some of them "might" help
Your CPU is under-powered for Editing HD Video, but you seem to have succeeded so-far with your other Projects, judging from your Youtube Videos. Very good 

A Fragmented HDD & too many Auxilliary Files "Can" cause the symptoms you've described. & a fairly recent nVidia Video Card Driver Update.
1) Check your HDD for Fragmentation
2) Go to; S14 File / Delete Auxiliary Files for your current Project in the Following Pop-up.
3) Delete the "Hidden" Render Folder. How to do that with XP; HERE (The Link to my Tut for Vista / W7 is missing).
4) Post a Screenshot of your Timeline at the point where the Ruler sticks. How to do that; HERE.
Studio isn't my main Editor these days. I only use it for that Editor's Post production.
More Studio Users need to chip-in here, rather than just viewing the Thread.