Hi Bob,
Just to encourage you, I've a similar problem with a Tek2236. It's smaller (trace starts a little late at fast sweep settings, the scope had other problems as well. Needed a power supply revision, so did that first. Thats fine now. Very interested in your findings on this one. Lately, I've tried to measure the horizontal trace issue, which is not easy to pinpoint.
Replaced a booster ceramic cap in the Z-amp output stage (have't looked at 465 schematics yet). Seems to change things a bit, but cap voltage of the one I used is too low (50V, need 200V), so I'm gooing to order a new one.