Hi,
I had the exact same problem yesterday. My first thought was that the power supply was giving up, but since my G27 is nearly new (played maybe 30-40 hours with it) I doubted it.
I disconnected the power a few times & changed outlets to see if it corrected itself, but no luck.
So I disconnected the USB instead & it calibrated like it should on first try. Then I had to download and install patches & my G27 went crazy again.
I disconnected USB once again & got it to calibrate like it should.
I had no more problems at all during my 4 hours of playing.
I don't know why it did it or what could be the cause. I've never had problems with it before & the only difference since last time I played is that now it was GT6 & I recently had to upgrade from my old fat PS3 to the newest slim model.
So, my guess is the slim model or newer PS3 firmware is to blame.
I Will spend time with GT6 tomorrow. If my G27 acts up again I'll report back.
Edit: typo