As an update.
The TMAP sensor appears to be producing duff information. When driving along the manifold pressure goes from normal up to full on (107, heh I don't know what the scale is) and then back down again. This causes one hell of a surge from the engine... repeat. Lots. The car is almost undriveable around town but it's better at a cruise. Not superb, mind...
Is there anything else that can cause this reaction other than a duff TMAP sensor or the wiring loom? I've cleaned the ICV, no effect (barely any crud in there either, heh).