I don't know which gearbox is in the DHSE, but if it's the ZF 4HP24 then it could be the same problem that you get sometimes with the ZF4HP22 box in the Classic.
On the 4HP22 this is often a symptom of the kickdown cable being adjusted incorrectly (too tight), slackening off the cable cured it on mine. There is a little metal crimp on the cable which should be around 1mm away from the cable's outer when the accelerator pedal is fully released.
If the cable is too tight the gearbox will hold gears until you lift fully off the gas for a second or so and change with a thump into the next gear. It will also change down a gear with the slightest provocation.
If the cable is too loose the gearbox will shift into top gear too early and eventually damage the brake bands.
Adjusted correctly the 'box should shift into top gear smoothly at around 45 mph and the torque converted will lock up at around 50-55 mph. I don't know if these roadspeeds are right for a P38 or diesel though, but it's worth a check.