Post Leopard Startup Problems

Having done all the usual erasures, new installs, reformatting, pulled RAM and hard drive out and in, zapped PRAM, NVRAM, pressed various key combinations ...It all boils down to this:

Press power button, whirr, chime, click click (drive, RAM?), no more clicks, hangs, black screen, no peripherals light up.

Then, minutes or hours later after many attempts to start, it catches, click click, click, click click and on it goes. But then it will usually crash at some point after that.

Hardware? Soft? Cosmic revenge?

All help is appreciated, thanks in advance.