*sighs and puts on the broken record voice*
What are your clips encoded with? you shouldn't use xvid/divx for editing.
is the new file anyof the following which can cause problems : encoded with a different codec, frame rate, AR, stored on a different drive, do you have enough virtual memory, RAM/CPU HDD space etc, have you got CCCP installed with Haali set to split the type of file you are using to edit with, really there could be a lot of reasons why.
did you shut down your machine, restart and then try again...
Have you read the sticky at the top of this forum?