Ford Escort LX TDI cutting out

My sons Escort TDI (1996 120k miles) runs well most of the time and always starts first time on the key. Sometimes however it will cut out completely during normal driving. It might run for several days with
no problem and then cut out several times in quick succession. It always restarts by turning the key off and then on again. Any ideas?
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Look at the cam or crank position sensors

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