Thanks for the reply claw, although I'm pretty certain that's not the issue. The craft is quite simple, actually. It's a mk1 lander can, with 4 large rockomax x200-16 tanks attached radially. I have a science jr on top with a docking port on top of that. I have a circular under slung battery pack, two stratus v roundeified RCS tanks, landing stRuts outboard, 2 goo cans and miscellaneous other utility fittings such as nose cones, RCS ports, thermometers, etc. the engines are 4 rockomax R55s. So, the only control authority aboard is the lander can and the engine gimbals. The engines are not situated perfectly radially, so maybe that's it? Either way, the thing shouldn't be convulsing and snapping its own back. I'm playing science mode, so I'm not sure if the "SAS abilities" you mention are active in science mode.