I haven't been following this discussion, but would recommend considering another possibility (if not already mentioned) that a gain stage may be oscillating at a very high frequency, adding apparent fuzziness to the trace. If this is happening in the vertical, and the BW limit has no effect, then it's likely past the BW limiter, like in the output amplifier. If it's in the horizontal, there may be sweep position or rate effects. Maybe lead dress or component position, or a loose grounding screw? I suppose this could also happen in the Z-axis, but you'd probably see it in the test point signal.
Ed
