Twice in 2 months (yes, rare) my 400E has developed a "lope" which cycles in rhythm every 2 or 3 seconds. It's noticeable when cruising, and has gone away both times when restarted after driving. The condition wasn't noticeable at startup either time, but developed after driving, and if I recall, both times I had just given it hard acceleration. New (reconditioned) MAF, o2, plugs, caps/rotors, coils, and coolant temp sensor (all done prior to the first instance 2 months ago). Fuel delivery? ECM?