Video Jumps when editing

Hi folks,

I have Windows XP profesional (all updates and so on, 
maybe too much:-)) Pentium 4, 1000 mbs RAM. 

When I import some video to Microsoft Producer, before 
throwing it into the Timeline I can see it perfectly. 
When I put the video in the Timeline to edit, it jumps 
regulary. 

could anyone tell me what could happen?

Thanks!!!

DAniel