Upon Startup, apple logo distorts...why??

I have an iMac G5 from late 2004. It is a 1.8 Ghz machine running OS X 10.3.9.
When I start it up, the apple logo appears as normal for a second or two, then appears distorted, as if the file is being processed with the wrong pixel X pixel setting? The iMac will arrive at the desktop normally, at the usual speed (no slowdown).

Also, perhaps related...

My iMac has been running well, but recently, I occasionally get a lockup (can't force quit, arrow is frozen), and the fans start roaring, and I am forced to shut down manually by holding the power button. The iMac will start up fine, at normal speed, afterwards. Not sure if this is related to the apple logo thing.

Any thoughts about whats going on here. Is this a warning of impending failure?
Thanks for any help and suggestions!

Hmm try the usuals:

Run your maintenance scripts, repair disk permissions and maybe clean out you drive.
See if some process is running that shouldn't be.

http://www.apple.com/support/imac/repairextensionprogram/

Sounds like your machine is one of the faulty batch of first generation iMac G5's...
Check to see if your serial number is covered and contact apple or your local apple service partner.