Firstly, just what sort of errors are you getting? As in, it crashes, or a black screen with apparrently nothing happening, or desynchronised voice, or even jutteryness in the video?
If crashes, you probably need to use different encoding settings. Try the moviepark tutorial over at www.GP32emu.com, or if you feel up to it (its simple in principle, but it took me a while to get right) the 2-pass tutorial available... no idea where. This also applies if the movie just looks like a blur or something - it may well be a bad encode, and re-encoding from either the DVD or a devent DivX version would be a good idea.
If you're getting a black screen, give it a minute or so. GPCinema is a lot faster at loading shorter things, so the longer the movie, the longer it takes to load. The gamepark won't respond UNTIL the thing has loaded so it may look like a crash - but the difference is with a crash it either resets or gives you Mr. Spiv's bios monitor screen.
De-synched voice is probably another type of encoding error, although as with jutteryness, it could be simply that you need to turn the clockspeed up a bit. I think It's "R" to do that. 133 is good for most things, but if the resolution is the full 320x240 and bitrate is particularly high then you might consider trying above that. If the resolution is 320x176, I've found that even up to 17fps, 120MHz is often fine, although it wouldn't surprise me if your experiences differed to mine.
Hope that's helped a bit. I suspect its the black screen thing, but I wouldn't swear to it; if it hasn't helped, then please give more details about the problems, and I or someone else'll get to it eventually I expect
