Hi mate,

Sounds to me like your hard drive could be on its way out, but before you put your hand in you pocket ive got a few things you can try.

***first off all on one the occasions it does start up back up all your data***

1) Check all the connections on the harddrive to make sure theres nothing loose

2) find out the hard drive manufacturor and download some diagnostic software from there website (if available) or get 3rd party diagnostic software.

3) replace the IDE cable if you have a spare one just in case one of the wires are damaged

4) reformat the harddrive incase of a software problem

Try these if you haven't already mate

post back after n ill see if i can think of anything else if you still haven't cracked it!

Hope this helps Dave