Startup/Hard Drive Failure on iBook G4

Question

My iBook G4, bought about two and a half years ago, recently developed a very serious problem. Occasionally, on startup, I get a blank screen, then a flashing folder icon with a question mark/Mac OS icon. This happens erratically. Sometimes the computer starts up just fine.
The first few times it happened, I did an Apple Hardware test, and got the following error code under the 'mass storage' heading.
2STF/2/3: ATA - 100 ata - 6 - Master
I rang Apple, even though the iBook is out of its 2 year warranty (by a couple of months naturally). They suggested it was a hard drive fault.
Foolishly trusting them more than I should have, I went out and bought a new hard drive (with identical spec) and replaced it myself. During the repair I noticed one of the screw fixings (a little metal loop) on the bottom shield had sheared off, but thought nothing of it (I find it hard to believe this has any relevance).
I replaced the drive, reinstalled everything, including all the old files, and the computer worked fine... for about three days.
The computer started to hang for a few minutes every so often when asked to do simple tasks (open an application, save a document, etc.) and we'd get stuck with the colour wheel. (I've read a few examples of this phenomenon on discussion forums, which is worrying). The applications didn't crash, they just worked very slowly, and it wasn't confined to any particular app.
Eventually we started getting the same erratic startup error. Hardware test reports no errors, as does Techtool Deluxe (even when I run every single test, including a surface scan). I opened up the iBook again to check all the cables and screws, but everything looks perfectly intact to me.
Has anyone encountered anything like this before? I am utterly stumped, and not nearly wealthy enough to have Apple fix it (especially if it's something as serious and costly as a logic board problem or something). I heard a rumour there is such a thing as a 'memory distribution' problem that might cause these symptoms, but google doesn't turn up anything under that description.
Please help! My Mac is dying!

Answer 1

When you installed the new HD, did you also reinstall
the OS, or did you transfer your existing information
to the new drive?
I reinstalled the OS and used Migration Assistant to bring back everything else. I guess I could try doing this differently, in case its some corrupt library date that's causing all the problems. I'll try that and post again in a few days.
Strange Disk Utility does not recognize your HD.
Does it only show your DVD in the left panel?
Yes. When I startup from the OSX DVD, there is only the DVD and a 'network startup' option, but no hard drive at all.
Go back to "safe mode". Once there, select System
Preferences>Start-Up Disk. Does your HD appear?
Yes it does.
Have a look at
this Apple article regarding start-up
problems.
Here too.
Thanks for these, but I think I've already tried all the relevant things, thanks to your advice.
Outside of repairing your HD via Disk Utility, or
replacing equipment (eg logic board) either Disk
Warrior or a
reinstall of the OS would be your
options.
Well, I probably can't afford either option, but we'll see how desperate I get. Thanks for your time.