I have the same problem, but a may be due to not having setup for flight properly (I don't know). I just purchased the 757 and took it for a quick test flight, nothing programed in the FMC. I've had other 757 so I know how to setup a flight. I did align the IRS and had the FD on, had it trimmed out for takeoff, but it may have been off. I do think It had to do with air speed (won't know until I try another flight). I set AP and it didn't happen until flaps were up. I didn't think of air speed until I read a post, so I don't know what the  airspeed was at the time.
I do know that after the aircraft got somewhere above 200kts the rocking stopped. So for the org. poster he may want to check airspeed.