2003 1.9 sdi intermittant revs sticking?

Hi, I have a 2003 vw caddy with a 1.9 sdi. The car started to intermittently go to a 1200rpm tick over on start up and stick there until re started. Then it would start and rev ok. Over time it started to play up when driving, sometimes when revs dropped to change gear it would do the same, so a quick turn of the key out then into ignition would solve the problem.. I have been told this is a fuel pump issue but my electrical experience makes me wonder if it is the crank speed sensor. Yesterday it played up a treat so today I set to work on it over a beer. My oscilloscope is mothballed so I cant check it in the conventional way so i set to cave man methods starting with unplugging and later removing it to give a visual. It seems the more i disturbed things, the more the engine played up and so the sensor is here on my desk now. Do these symptoms seem to match a failing sensor or are they just coincidence? Any thoughts appreciated..