Yep. AP1/AP2, basic autopilot, and Altitude Hold. She's been doing this since she came to us from Patrick in '02 or early '03. It starts oscillating, small at first, then worse...almost like it's missing the servo feedback. No associated bit faults, wiring Ohm's out great, op checks great on the ground. We write it up for an inflight check and she fails every time. We've gotten it to go away for as much as 6 months by changing the elevator pack. A 7-lvl flew with the aircraft--when the aircraft would porpoise the elevator trim indication would follow the porpoising, which it should since the autopilot uses the elevator trim to assist the surface. If the crew put an OPPOSITE input to the flight controls the porpoising would null out and stop for a while.
Getting to the point of calling for a depot team. Or putting a grenade in the drybay.:D