I was thinking about this the other day and this post reminded me.
What happens if you're stopped, in 4LO and you drop it into neutral and hold the revs at 2000 with the accelerator? Does the engine still fluctuate/surge? If so, it would be interesting to find out what the turbo actuator is doing at that point in time. Pop the bonnet, have someone hold the RPM and look at the arm beneath it.
What if you're in 4LO, 4th gear, clutch depressed, holding the RPM at 2000? If it surges then, could the turbo be looked at? I'm just wondering if it's a signal to the turbo that's being confused.
Last, it doesn't know what gear you're in. What if you pull the plug from the gearbox? It's the one slightly forward in the pair on the right hand side of the gearbox. My thinking is that since the gearbox only signals whether it's in gear or not, if you pull that plug, does the surging stop?