What renderer are you using? The standard render still has issues with motion blur (errors, blur on objects even if they are still, a blobby unconvincing blur). If you need to use Standard for speed, render the motion blur as a separate motion blur data pass and composite it in AE with a blur filter that can accept that blur data pass (Real Smart motion blur for instance).
Otherwise use the physical renderer - its slower but does a better job on the blur and does it all in one. Check that your physical camera's shutter speed is not keyframed or that the blur level is not keyframed.
What also could be causing the error is if something transparent or reflective passes in front of the object.
It could also be keyframes nearly overlapping. So each frame renders properly but the motion blur is being calculated over a number of frames and the space between frames. If a keyframe that may not appear to render (because it lies between keyframes) is present and of a much different position, it could cause the blur to suddenly "warp out".
You can quantize your timeline keyframes so each keyframe lands on exact frames.
Or it could be the object gets too close to camera and the blur happens "inside" the camera (closer than the cut off for the objects close to camera)