peugeot 406 engine surging / hunting

I have a 1996 Peugeot 406 2.1Ltr GLS TDI, the engine has started to rev of its own accord. It somes surges between tickover and 3000rpm and once it has gone right round to 5000 rpm and stayed there, the only way to stop it is to turn the ignition off. On restarting it is then ok for a day or so. Any ideas would be most welcome