Maybe the autopilot is struggling to hold it straight because the 707 is too "fat" to be at that altitude. I know when mine did it, my airspeed was about 248 KIAS. I know I wasn't doing 250 KIAS because I was using full throttle the nose was slowly settling down as my speed crept up to and above 250 knots. It never got fully stable until I started my descent.

Mark