Engine cuts out

95 VW Caravelle 2.5 petrol manual. Had the clutched fixed (works great now) drove it home in the evening about 10k and it just stopped. Plenty of battery, but would not kick over. NRMA (Sydney) arrived after about an hour, and advised the Crank Sensor or the Security (key) may be the issue. Swapped the key for the spare (had to get home and back) and it started. SO it looked like the key was stuffed. BUT next day same thing: aound midday drove about 2 K's - using the 'good' key, and it stopped again. Luckily at the local shopping centre. Spent 2 hours shopping came back and it started first go. This leads me to believe it has something to do with a heat sensor, or a sensor that is faltering when it heats up, (hopefully not the computer - this has been fixed twice in the 7 years I've had the van. Plenty of water and oil. Anybody come across the same or similar issue. Thanks.