My 740 (non-turbo, automatic) suddenly has begun to jerk back hard whenever I let of the gas at higher speeds. It also jerks a little when I re-apply the gas. This only happens when I'm up at 50-70 mph. Above or below there, it accelerates and decelerates smoothly. I've tried letting off the gas very slowly, but at some point, I get the bump. It doesn't appear to be down-shifting (the rpms stay the same throughout), just a very distinct "bump" vs. smooth decel. My non-Volvo) mechanic said the axle carrier bearing was 'sloppy' and thought that might be it, but $400 bucks later, no change. Any suggestions? Thanks.