Get a reference number for your calls.

Phone them EVERY time it happens.

Insist at a 450 mile round trip is out of the question.

They will either (a) have you ship it in for repair or (b) send someone out to look at it.

I would take a look at a small program called iStat. This will show you the internal temperatures of your G5 and may give an indication as to why the fans are running full bore.