random screen shut offs?

About four weeks ago I started carting my ibook to and from school so it gets more jostling than it used to. Today it shut off twice with no warning, then the third time the screen went nearly black though the computer stayed on. Is this a common or known problem? I'm sure it's not worth the cost to fix either way, but I'm wondering if I need to budget for a new computer in the next couple days or if it's likely to make it a bit longer (I removed and replaced the battery and it turned back on; it's working for the moment but this was just about 20 mins ago.)

Which iBook is it? G3 or G4? What's the processor speed? Does the screen flicker?

If its an iBook 500 or 600 it might be a Reed switch problem which is a known problem.