Doom PSP Problems

Well, I downloaded the Doom PSP Shareware homebrew from psp-hacks.com. But I recently bought the doom collectors edition with all 3 dooms. So I read before that people just put their full versions on their psp so I decided to take the wad file from doom ultimate and put it on my psp. Well it loads and all but after a few seconds in the menu screen, it will freeze. And if I quickly make a game, it will be alright for a few minutes but after awhile it too will freeze. One time it happened, i noticed it said Help 2 file was not found while in game but I don't know. Would anyone happen to know what I should do or make any suggestions? Thanks

well, my best guess that it might be a problem with the .wad file, but that shouldnt be a problem....did u name the ultimate doom wad DOOMU.WAD?