Car will not start sometimes

I have the same trouble 02 Mercury Sable will start day or night cold or warm then all of a sudden will crank no start replaced cam sensor still no start cranked few more times then had a start . When running runs great. I even hit it with starting fluid to check if it was a fuel problem still no start . Tried everything Would love some imput on this

I also have an '03 with same intermittent problem. Starter fluid works for me. Cause is a mystery. Does anyone have a solution?

Try replacing the starter motor relay. In the Sable it's #23 in Power Dist box under hood. About $15.