Macbook takes 3 attempts to boot

Hi all

Macbook
1.83 Ghz Intel COre 2 Duo
512 MB 667 MHz DDR2 SDRAM
Mac OS X 10.4.11


Recently experienced Macbook shutting down mid use, as well as issues booting up ( machine would sound as if it was about to boot up but would end with a click before the grey screen appeared. On other occasions it would make it to the blue screen and then shut down. )

I've replaced the hard drive and saved all info and pics etc. Reinstalled the system discs and all was well for a few days.

Now, occasionally it will take 3 attempts to boot - it makes a 'winding' sound ( which it normally makes in a healthy boot up ) but it will make this sound 3 times ( sometimes more ) before it actually decides to boot.

I've tried resetting the NV ram and resetting the PMU also. Tried turning it on and off a few times during the day but the problem is back.

Any clues?
Thanks in advance!
Jt