Quad G5 - Fans running > 3000 rpm, CPU B overheats to 100 degrees

This machine is now about 2.5 years old, and the fan noise has got louder since I bought it. The last few days, the fans have been blaring away at full speed. I installed Temperature Monitor, then to get better info, iStat. Right now, with only one Safari window open, CPU A is showing two temperatures below 35 degrees centigrade. B is hovering around the 60 mark, i.e. double. The CPU is reporting 97% idle.

I opened the case and cleared out all the dust which had accumulated inside. There was a fair bit. I generally open the box every 3 months and clear out. This didn't make any difference whatsoever.

Thsi afternoon, things got worse. The system began going to sleep while I was using it. I checked the logs, and they were showing CPU B hitting 99 degrees centigrade and forcing sleep in order to save itself.

Up until last week, I had the performance of this box set to Highest in energy saver, and never turned it off -- only rebooting for the rare crashes (like 3 a year, at most) and software updates. After seeing my most recent electric bill, I changed my settings to Automatic and set the Mac to witch off at midnight and start up at 8am. Apart from that there has be no change in usage, nor have I installed any new software or hardware.

Following the forced sleep, which happened this afternoon, I have set performance to Reduced, disabled the screen saver, told to computer to auto sleep after 30 mins inactivity, and sleep the display after 15 mins. I have re-enabled the setting to sleep hard disks when possible. I even used the processor pane from CHUD to disable cores 3 and 4, but this seemed to do very little for the temperature of CPU B, so that's been re-enabled.

Following other advice, I have rest the PRAM and NVRAM, reset the SMU, and removed every device from USB and FireWire bar the keyboard, mouse and monitor. I ran the Hardware Test in full. It took over three hours but reported everything as OK, RAM, Logic Board and GPU. Disk Utility verified both hard drives as fine.

I am dreading hearing that CPU B is failing, and find it hard to believe a machine less than three years old, sold as a professional workstation, which cost me £3,000 and took me two years of a very expensive loan to pay off, would have this kind of catastrophic failure.

Is this going to cost me a lot? Have I just been taught the hard lesson that you don't get what you pay for anymore? I love my Mac, but if it costs me the price of a new PC to replace a part, I'd be very upset.

Can someone recommend any further diagnostics or a possible explanation and solution? Do I just need to bite the bullet and get to the local Apple Centre?

Thanks in advance for any help you can provide.