IMac :: HD Fail ... Weird Boot Ups?

my imac boots normally unless three things happen prior to it

1. i put the computer to sleep

2. i schedule a shut down via energy save

3. my computer undergoes a forced shutdown e.g. via power button or power failure
if any of these occur i get the infamous flashing question mark, however if i wait 30 mins it boots normally again, is there a reason for these problems other than a failing HD and can i do anything about it

f.y.i. i have already attempted clean/fresh installs of leopard and snow leopard