iMac 2008! PVRAM/NVRAM or HDD Issues?

[a9205]

Hi guys, basically last october i started to get the folder blinking with a '?'
i presumed there was a HD problem but after numerous smart and diagnostics test and the fact that the mac has still been working for now 6 months im wondering if the issue is the 'logic board battery'.
once or twice a week when i turn the mac on i get the blinking folder. As well as when i try to boot after i have put the mac to sleep or a scheduled shut down.
recently on a few occasions after i have got the blinking folder i have restarted and reset the nvram and the imac booted up.
so i was wondering if the issues is in-fact with the or logic board memory.
In case you were wondering the reason i haven't replaced the HD is because it is over £130 and i don't want to waste it buying a new HD especially if the issues wont be resolved.
thanks

[MacVibe]

My 2009 imac is flaky like that too. If it was repeatable I would take it in to get serviced because at the moment it is still under warranty. Once I had to restore from time machine. Typically when I can't get it to boot I have ultimately been successful by disconnecting the firewire disk drive.

Conventional wisdom is that it is better to fix the blinking problem sooner rather than later. So first, you could do an OS reinstall. Second, if the problem keeps popping up you may have to bite the bullet and get a new HD.

Edit: Also, to minimize my problem I have disabled sleep. I really hate this because of all the wasted power but it has resulted in a much more stable system.

[Hellhammer]

System Preferences -> Startup Disk -> Select your OS X disk/partition -> Reboot from it

If that doesn't help, give my guide (http://guides.macrumors.com/Mac_doesn%27t_boot) a try

[a9205]

but do you think there may be a problem with the logic board battery?