Andy,
Basically what I believe is happening is the A700 sends an electronic signal to the lens in order for the AF motor to activate. It would seem the Sigma 70-300mm cannot deal with with the high torque signal from the A700 thus the gearing inside the lens (the teeth/cogs) basically wear themselves out so they do not engage one-another any more, or if they do . . just barely so you get the lens juddering away like crazy where the gearing teeth are trying to engage each-other.
So yes . . . knacker the AF motor in the lens.
Either way, it only took 2 hours after first use for my lens to become a victim of the phenomenon. I thought it worth posting a separate thread due to the nature of the problem.
The A700 has a new high torque motor in the body for improved AF speed on all (excluding HSM) lenses.
The motor in the body uses a "screw drive" to the lens to focus the lens. It's the internal gearing on the lens that has been "striped".
The issue appears to be that the cheaper Sigma lenses seem to have used components that are not up to the torque generated by the A700, so the A700 AF motor strips the plastic gearing within the lens. Changing the AF speed to "Low" reduces the torque generated by the body.
This only seems to be an issue on the cheaper Sigma lenses, since I guess this is where Sigma have used lower quality components. I have also read that the issue also existed with the Film Dynax 9, which also had a fast AF.
I know it doesn't really help, but I would be interested to see Sigma's response. They basically need to upgrate these components to fix the issue once and for all. Repairing with the same components isn't really a fix.