Does it crank at all when it won't start? Or does the starter just not spin at all? You might also have a bad starter solenoid. I have had a couple of Fords that had bad starter solenoids... If the idoit lights all light up but the starter does not crank with the key in the start position, this would be my first assumption (or, possibly, a bad spot in the starter). You can try jumping the starter solenoid with a screwdriver (just use one with a thick plastic or rubber handle!) next time this happens to see if that indeed does the trick.

Chris

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)