Hi mate, re the revs surging, worse when hot. Had this happen just about a week ago on my 4b, was dirty sensor connections of the 5th injector, the cold start injector. First off I took the car to my mechanic, who disconnected the 5th injector, said it wasn't really needed, would make it run a little rough when cold maybe, but after this the car would sometimes stall and refuse to start (was flooding), called the RAC and got lucky, the RAC man had an old RS2000 with a Cosworth block in it so he knew his stuff, he figured that the 5th injector sensor plug just needed a clean and he could reconnect it with no problem. And all was well without me spending anything