224MB is an odd number, sounds like 3x64+32 or 128+64+32. If all those chips aren't the same memory type and speed, it could cause trouble.
If you get additional RAM, make sure you only use matched chips.
Very good advice for this problem.
This site seems to break these problems down pretty nicely:
http://www.backupbook.com/03Freezes_...0.default.html