Charged the battery and tried again with the 35 L and the results; exactly the same as before. 12 fps H and 8 fps L locked at exactly the same thing equally quick and they both lost their ability to lock at the same amount contrast and light. One shot still worked until too dark to see and beyond.
In 12 fps H mode it locked on the black zipper of my black Induro tripod bag with f1,4 @ 1/60s 6400 iso. I think that's pretty crazy... Shot with center point, no expansion.
My serial number is: 053011xxxxxx
Remembering that my main issue wasn't even in remotely low light - it is a strange issue with seems to effect a fair few bodies. I can make it fail in daylight - i can just about predict where it will fail at 12 fps.
I spat the dummy with mine - I have had it die on my twice in one wedding shooting in broad daylight - two days ago, one shot - 70-200 f2.8 II with 600ex rt on board (fill flash) - iso 100 1/600 at f4 - lens locks up and refuses to rack, had to turn it off and on again to start working.
Canon are replacing it tomorrow when I get back to Australia from overseas, however after not hearing back from them for 4 weeks I had to threaten to dump my Canon gear to get their attention.
The most annoying thing is even the head of Canon Australian and CPS Australia won't comment on the findings. They are basically ignoring the issue... it's disheartening.
I'm not sure if you have noticed there was a report of olympic shooters having issues during the games where canon reported 5-6 months to fix the af issues...
On a side note - we were in a reception with 5D2, 5D3, 1D4, 1Ds3, 1DX and 7D. The 5D3 and 1DX consistently had issues locking onto dark skin (not so good if you are working in asia). 5D2 actually had very little issues. on the 1DX I had to actually manual focus to get any shots which to me is rather stupid.
Using the 600EX for af assist did help (and didn't lock up when using at EF 50 f1.4)
I will have to redo all my tests with the new body tomorrow.