PowerBook 15" - strange problems

PowerBook15, 1.67, 1gb, 10.4.2

Left to sleep, when woken it's twice gone to a blue screen, and remained there. Mouse works. Rebooting doesn't help - it just goes to the blue screen at the end of the booting sequence, before the desktop loads.

As I say, this has happened twice; each time rebooting holding down the shift key - and then rebooting again - restored things.

But yesterday it had a totally black screen, not a blue one; but, after repeated reboots, normal service was resumed...

Today the screen had automatically reset brightness to zero, and the 'brighter' key didn't work. It took much squinting and prayer before I could activate Preferences, and reset the brightness...

Any ideas as to what's happening? The machine is still under warranty, but, as it's now working perfectly it's hard to see what I can ask AppleCare!

- duncan