I have the same problem when I listen to MP3s of radio broadcasts (which are usually 2 hours long).
Trying to fast forward or manually moving the slider just freezes the system.
The ultimate solution would be an app that could remember your place. So if you closed Media Player it would just start playing the same file in the location you left off. So far, I don't know of such a media player for PPC.
My solution was to split up the MP3 into smaller, 30 minute parts.
Take a look at
MP3 Splitter. You can choose how long you want the segments, and have them numbered so you can remember which part you were listening to.
Not the best solution, but it worked for me.