Wobbling on a straight section of track

[progferbrains]

Hello

I have a straight section of track on which all trains at all speeds wobble as though the track is hurling them from side to side. Very weird.

Here are some background facts:

1. It's Wear Valley Railway Track - correct track rule in play
2. Uneven-ness is set to zero
3. There is no superelevation
4. It's double main line track (not yard)
5. It has a gradient of in in 80
6. probably the most important... it was a replacement section as it previously had an annoying un-needed bend/kink in it.
7. There are no orphaned sections of track
8 ALL trains do it from DMUs to LSWR tank locos and coaching and freight stock responds similarly.
9. Only ONE section of the track on the whole route does this and there have been other straightening replacements

Any ideas? Anyone? I mean - I tried to re-lay it and the same thing happened
Thanks in advance
Andy