iMac loses iCal and iTunes. What's next?

I got my G5 iMac back a few days ago after my local Apple Care people fixed the video issue. It worked fine for a couple of days but has now started to misbehave. Yesterday I lost my entire iTunes library (and failed to retrieve anything despite following instructions on other threads).

I then downloaded Senuti and restored those files from my iPod... but this morning, iTunes is again telling me my library is damaged.

One restart yesterday also saw the dock reposition itself along the bottom of the screen, so I patiently put it back to the size and position I prefer. On start-up this morning it was back along the bottom and, more seriously, my iCal had been wiped clean of all data. My address book and iPhoto files are untouched, but for how long?

I tried restoring my calendar by synching it with .Mac account, but this failed. I got a message along the lines of 'come back again when you've got a computer that works.'

Is this related to the warranty work just carried out, or is it a coincidence. More importantly, does anyone have an idea of how to fix it?