Mac Pro sudden shutdown

Question

Hello,
anyone with a similar problem? I found related threads but in most reports nothing was solved. My MacPro is completely unusable now.
Description:
After initial startup the machine runs for 20 minutes (+/-), shuts down without warning then reboots. Typically does that again then just dies. Power LED remains on but machine must be unplugged from mains then plugged in again before the power button will work. Nothing in the console log points to failure, just the standard startup first entry 'hi mem tramps at 0xffe00000'. No crash logs are found related to the shutdown.
My local service provider has been trying to fix it for nearly a month. They replaced the hard drive, the PSU and memory sticks with no result. Now they think one of the memory risers is faulty and are waiting on a replacement. They cannot give me a clear reason for any of these "solutions", they're just trying things to see what sticks. So if you had a similar problem with shutdowns please pass on the details!
Spec:
Machine Model: MacPro1,1
Processor Name: Dual-Core Intel Xeon, 2.66 GHz
Boot ROM Version: MP11.005C.B04
SMC Version: 1.7f8
Related threads:
http://discussions.apple.com/thread.jspa?messageID=4699492&#4699492
http://discussions.apple.com/thread.jspa?messageID=5154970&#5154970

Answer 1

Only two reasons I can think of why it might suddenly shut down.
1. Overheating. Check to make sure the system isn't clogged with dust. If it is new, then a processor or heat sink could have come loose. Look for anything that feels loose with heatsinks.
2. Power cables that are loose. Also in shipment cables that tie power to the boards can work loose. They then become intermittent.
A good service provider can look for all of these things. Sometimes just reseating the cables and looking for loose items can fix things.
A really bad software bug can reset the system as well but I'd doubt that from what you're reporting. To check that, re-boot with the DVD s/w install disk. Then just let it sit. It should never crash and you can see if it is stable.
Also, try one of the s/w programs that reads the sensors in the system. They can look for temperature problems at the various sensors and point out a problem just before they cause shut downs.
Hope this helps!