I have a toshiba P105-9312 laptop. I have removed the motherboard from the system and have it tethered to an external monitor, power cord, USB keyboard, memory and HDD hooked up only. When I boot the system, 9 times out of 10, the toshiba post logo will be scrambled (readable but off center and the graphics are scrambled). 1 time (very rare) the laptop will actually boot normal, and run for hours, but as soon as I reboot it, back to the scrambled graphics and cyclic rebooting. Trying to figure out what to replace.

Graphics chipset.

GPU (Graphics Processing Unit)

http://www.irisvista.com/tech/laptops/toshiba-satellite-p105/take-apart-3.htm

Click on the photo in Step 20.
GPU is the second chipset down. The one under the Processor.
(CPU = Processor)

Photo in Step 19 shows the cooling module removed.

(Cooling Module -
1) black Fan Assembly,
2) copper Cooling Tube,
3) heatsink plates attached to the Cooling Tube,
4) Finned Heatsink. {Opposite side of Fan Assembly/Cooling Tube at top left of photo)

A) It could be that the thermal paste has dried up on the Processor, GPU, or both.

Heatsink plates for Cooling Tube need to be thoroughly cleaned on the bottom. Processor case top, and top of GPU need to be thoroughly cleaned, and new, fresh thermal paste applied properly.

B) It may be a problem with the BGA surface mount that the GPU is in.
Heat may have loosened the solder joints creating a poor contact.

BGA surface mount:
Using an older Intel Pentium 4 processor for comparison.

The Processor has contact pins coming from the bottom.
These pins (478 of 'em) line up with socket holes in the Socket 478 processor socket.

With the BGA surface mount there are no pins.
Solder Balls take the place of the pins.

There are no socket holes.
Copper Pads take the place of the socket holes.

The chipset is set into place, (In this case a GPU), with the Solder Balls lining up on the matching Copper Pads.
Heat is applied at a specified temperature, and length of time.
The solder balls melt thereby soldering the chipset to the copper pads, on the motherboard.

Overheating of the GPU can cause these solder joints that were created to partially melt, then harden.
This results in a cold solder joint, and a poor contact.

View this type of problem with the Pavilion dv2000, dv6000, dv9000, and other HP models, and the solution one tech came up with.

http://www.youtube.com/watch?v=bnkQNmKauEc&feature=pyv&ad=5823851286&kw=DV9000

For additional questions or comments, please state in a Comment.

Regards,
joecoolvette

Probably the in build graphic chip is defecting.. try to locate it and apply little preasure with your fingers while starting the mobo. if success - then you can try to rework/resolder it, but special tools are needed (hot air soldering iron, or infrared rework station..)