try getting a laser temperature gun and take various readings around the motor
i just had a thought
could this possibly be a partially clogged head or block?
If you see abnormal hotspots in the head or block that might be an indication.

You could still be getting some circulation in the radiator because of the bypass circuit but if the head is clogged partially it could cause this problem.
I'm not discounting edwardst's suggestion because it is a high probability, in fact take temp readings on the tranny too, i just had this thought while sitting here brainstorming.

***edit***
Ok i got caught up in the rheteric and forgot he already said the trans temp is running at 230deg as well...
So does the engine cause the trans to heat up? or is the tranny heating up first and causing the engine to overheat?
MEngr does have a good point about the inching spool adjustment also, although one would think that this would cause a noticeable difference in drive performance if its heating this badly?

It may ultimately require you to start pulling systems apart and examining them more closely to find this problem.

Just a thought..... thinking outloud here ;o)