You've got something going on that shouldn't be. Find out what's killing you (memory, rouge process, etc), and then figure out why it's doing it. look at the process list, via top or ps, and see what's eatting lots of memory or processing speed.

Good odds you installed a bunch of crap and it's running something in the background, or something has a memory leak. My N900 is up weeks at a time (uptime is 15 days right now), and I have several things loaded (including K49 & CSSU). Still not sure, check out this post.