Dunno Tim, could be lots of things. Did you blank the end of the vacuum pipe. Some of the later cars were a bit more electronic. Perhaps the EGR is being watched and now it thinks it's faulty..just guessing.
A more serious thing is that oil from the turbo seals can feed the engine and it can rev out of control. Mine failed in a bigger way and fed the sump oil into the intercooler up to a level where it blocked off the air supply, thankfully because I've a pal who suffered this type of failure in a queue on a dual carriageway. The engine ran away with itself and couldn't be stopped. He got out of the Disco in case the flywheel came away and he left it to rev flat out until it consumed the sump oil. Apparently, the was a smoke screen for miles.
The Disco had only covered 30k, too.