I'm just curious but are you holding down the focus button while you shoot? It sound almost like you maybe pumping the focus button or letting off of it when you acquire a lock in the beginning.
If not I would try the single focus point approach and test again. If it is still not locking focus and holding it then you need to send it back to Canon.
I held down the shutter button half way to obtain focus and then held down the shutter button fully while taking the burst shots. In this sequence it had focus then lost it and stayed lost. In another burst I took it had focus then lost and regained focus several times in the burst from shot to shot. These bursts were taken with a single AF point with the surrounding expansion points on.
