Does this happen on any file, or just on certain files? Are the files it crashes on large, complex files?
I ask, because 512K isn't a lot of memory with you're running InDesign and handling large complex files.
Also, Apple's hardware test disk, while helpful, is not totally definitive. For that you need testing on specialized equipment.
Also, have done the usual OS maintenance routines, i.e., repairing permissions and checking the directory (with Apple's Disk Utility, or with a third-party directory repair utility, such as Disk Warrior)?