Macbook Pro will not boot from any source after upgrade

Question

Hi,
I have a late 2011 Macbook Pro - standard issue other than more memory as I recall. It has a 750MB hard drive with more than half still free. All the checks given below are done with the computer stand-alone with no hardware attached - other than the power cord and of course WiFi.
2 weeks ago I upgraded to Yosemite having taken all the right precautions before upgrade - thanks to Take Control. It worked fine and I was getting a good experience until last Wednesday when I put it to sleep by closing the lid. Later in the day I opened it up again but only got a grey screen. After leaving it several minutes to see if it cleared I shut down by holding the on button for several seconds then tried to re-boot. The computer starts and gets as far as the grey screen then shuts down again after several minutes. Sometimes it will continue to restart after shut down and keeps doing this until I again hold the on  button in until it finally shuts down for good.
I have two bootable drives - one is a cloned backup of the previous version and all the data, the other is an 8GB thumb drive with a bootable version of the Yosemite installer. For the record I also have a Time Machine backup and a second full clone backup and recovery drive on a Synology NAS device -  I just haven't figured out how to boot from this device as yet.
I can get to the boot up selection screen (hold option key) and select an external drive to boot from but it makes no difference - I still get the same grey screen. Same thing if I use Command +R to boot from the Mac HD recovery partition.
I have also tried to boot into safe boot mode (hold the shift key down). This seems to slow everything down but ultimately has the same result - a grey screen.The only difference I can see is that this grey screen does not shut down after a time. Seems to just continue until I shut it down manually.
I can hold Command +V to remove the grey screen and see the action being written to the screen while logging on (white text on a blue screen) - up to a point.  After some time I just get the same grey screen and it shuts down after more time. I can also use Option +D (I'm told it should be just hold the 'D' key to do this but that has never worked for me and Option +D does) to get to the hardware check and I have run this both before upgrading and after having the problem. Before upgrading I got one error - 4ETC/1/40000002. I looked this up on the Internet and could find nothing specific to identify the problem and the general consensus seemed to be it was nothing major. After upgrading and running the hardware check again (after this problem occurred) it returned no errors.
I have reset the PRAM with the same result as before - all I get is the is the Apple logo and a progress bar that increases to around 50%, then both disappear and I get just a grey screen. After a time the computer shuts down and then restarts again and goes through the same process.
I've done pretty much everything I can think of and find in books and on the Internet. Hoping that someone out there will find this problem interesting and help me to resolve it.
Tony

Answer 1

Problem resolved. I took the machine into an authorized repair facility where, for around $30, I had it checked and the logic board replaced. Picked it up yesterday and seems to be working OK. No cost to replace the logic board even though the machine is over 3 years old. There seems  to be some sort of dispute with Apple over a class action that has worked in my favour. Great. I'm happy.
Tony