When you say it kangaroo's do you mean whist pulling away or do you mean it idles irratically?
If it runs at ~800 and then drops to ~400 it will sound like its struggling to idle but it should not cut out. I would take a guess that if you blip the throttle it will pick up OK? If so I would not worry too much, its probably the cars management system having a moment! Equally it could be a sensor etc, I would monitor it for a while before changing the plugs again as I would say they are ok based on the time they have been fitted.
Oh and most Sups are now suffering from valve stem seal issues now...they are old cars remember! Again I would not worry too much unless its absolutely chucking out smoke. Keep and eye on your oil levels though!!
More concerning is the poor pick up on the 2nd turbo. My knowledge here is sketchy but I believe its normally attributed to the actuator, may be worth searching for a thread on this issue.
Good luck with it...