OT: VW Parts?

Certainly not a classic, but the knowledgebase here is far greater than in the wider community...... Big problem with our 1994 VW Passat 2.0 CL. Baically, the engine cuts out randomly at any time until it's fully warmed up. It can go for weeks without any problems, then fail every time the engine is cold or not fully warm, day after day. After anything from 20 seconds to about 5 minutes the fault will disappear again. leaving no symptoms to debug. I have never had any kind of problem with a fully warmed engine. The failure is sometimes but not always preceded by the rev counter jumping wildly or dropping to zero. For this reason I have drawn the reasonable conclusion that the problem lies with the ignition system. Searching far and wide on the net I have found regular reports of problems with the Hall effect sender in the distributor. While I would like to test this, the intermittent nature of the fault means this isn't possible - as soon as I have even the multimeter to hand the engine bursts into life. I am having problems finding a source for the sender, VW does not have it listed as a discrete item in the parts catalogue so I cannot find their part number. Can anyone suggest a possible supplier for this component, or otherwise suggest an alternative cause for the failure? Cheers, Bill.