Newly repaired iMac, crashing apps, no sound and me

[Unorthodox]

My iMac just came out of the shop because it was shutting down all the time.
They replaced the logic-board and the power supply.

Now some apps will crash as soon as I open them (including Halo!)
And others don't have any sound.

I reinstalled Tiger using the archive and install feature.
All the problems still exists.

After I reinstalled Tiger I created a new user account to see if that would help.
No luck.

Now I'm suffering from Halo withdrawal!

Edit: Oh, yeah. It's a rev A iMac G5.