PSP Movies-- freezing on start

Hi,
I need help with the movie player in PSP, i converted a DVD and a mpg into mp4 using 3gp once and video9 for the other one. using two pc´s saving time and to check out which is the better prg...
I put the files in the created folder and everything was working fine.
I had the thumbnails on the PSP Mainscreen, and the movies were working perfectly. I could use all the functions of the player during watching the movies.
About two hours later i want to give it a try again and take a look at the movies again, but know when i start the movies the PSP freezes and i just have a black screen and the play symbol is shown in the lower left corner.
I dont know what to do anymore.. :mad:
I have 1.50...
does anyone have the 1.50 firmware update and can provide maybe a new installation will help?? do you have any experiences in this? :confused:

Help would be higly appreciated..