In the end, it turned out to be some shape edited floors, far outside the crop boundary, that were causing the problem. Once I had added these to a saved Selection Set and then hidden it off, the zooming was smooth again.
A little story about Diagnosing the problem:
- Checked in other plan views - these seemed to zoom ok
- Changed View Templates
- Changed View Type
- Switched off Worksets until the problem was corrected - thus found that problem elements were in Workset1
- Turned off all Model Categories - problem corrected. Now, which category is causing the problem?
- Turn on about 10 categories at a time - when the problem reappears, you just need to narrow it down a bit further
- Problem category was Found - Floors
- Now, go to a 3D view, isolate Floors and try to figure out which element is causing the problem. In my case, I could safely hide off all of the shape edited floors.