Crank shaft thrust bearing check for end float in crank shaft it should not move backwards or forwards with a lever. I don't understand but this is what my husband says he is a motor mechanic
Is it a Mondeo by any chance? friend of mine had a similar problem on his, and it was caused by crankshaft float as has been suggested. Everytime he pressed the clutch or cornered hard the crankshaft was moving slightly and the crank position sensor was losing its signal, when he lifted off the clutch the crank returned to its correct position and the car restarted instantly!! Its a known fault on Mondeos of a certain age.