Can anybody explain why this is happening? I have tried to sort this out but have run out of ideas.

In the above render the chassis to the left renders how I would like but is not yet UV mapped.
The chassis on the right I have begun to map but as you can see, there are all kinds of weird smoothing issues that I just can't seem to rectify. I don't remember coming across this problem before.

Adjusting the smoothing angle does little to change things unless it is down to zero but it then becomes way too faceted. I have also tried to remap it through UVMapper but the problem still persists.

Smoothing is also off in the render settings. Adding edges to help does very little just changes where it goes wonky. The only thing that seems to have worked so far is using edge splitting but as the complete model has a quite high polycount I don't really want to go down that route.

Any advice would be greatly appreciated.

looks like the OBJ Artifacting again...

see the links here

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2762879
http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3388247

other than splitting or microbeveling, we did'nt find much of a solution :(
tho.. thinking about it.. you could try rotating the UVseam to a hidden area.... it may hide it..

my 2cents.  UV mapping should have no effect on the smoothing.

I dont see the smoothing issue you mention, but I dont know what its intended to look like either.
Unwelding some endcaps and selected points may help. 

I would finish mapping and see what you get.

A wireframe view would be good. I usually unweld any flat areas, Poser 5 tended to mess them up trying to smooth them. 

Mark, read the 2 links I posted.. we found there is often a smoothing error at the UV seam :(

I agree that mapping should have no effect on smoothing but just as a test I just unwrapped everything except where the wood and chevron markings are and just scaled them down to a small area on the map. As you can see the bottom model which has been unwrapped totally, the weirdness has gone (except in a couple of places). So I assume that unwrapping must have some effect.

I'm still going through the above links.