Videos chopping, but only when sound is on.

The video is chopping, but only when sound is on, if the track where the sound is, is muted it run just fine, tried to switch the sound with another one from a diffrent clip, but the results is the same, the video is still chopping.

When exported the videos is also chopping.

The computer runs

Windows 7, 32 Bit

Intel(R) Core(TM) 2 Quad CPU   Q8300   @ 2.50 GHz

                       2.50GHz

4.00 Gb (3.00GB can be used)

Some one have a solution to get it play right, especialy when exported.