Chosen Solution
I also get some flickering of the display
Ouch! This is a symptom of a dead discreet GPU. Carefully disconnect your battery following this guide MacBook Pro 17" Unibody Battery Replacement to Step 3. Now press the power button with nothing plugged in for a good 15 secs. Now plug in your battery again and your MagSafe charger, restart your system but press the T key that will force the system into Target Disk Mode. You should now see the FireWire Icon on your screen do you? The system should be stable in this condition.
