so you only felt the shudder while coasting? check the lugnuts, make sure all are torqued down tight. sounds dumb, but it has happened to me. quick fix!
did you notice any shuddering or vibration when braking? if so, then check your rotors/calipers/pads.
are you sure this shuddering isn't fuel related, seeing as you probably still haven't found those electrical gremlins?
