Ford Taurus 2001 speed sensor

I have a Ford Taurus 2001 SE. Every 4-5 weeks when I am driving the speed sensor goes crazy and 10 minutes after the check engine light starts blinking showing a missfire. This happens for 2 days and then goes back to normal for another month or so. What could be the problem? The mechanic told me that I should change the speed sensor but he says that this shouldn't be the cause for the missfire light. Anybody knows?

what do you mean by goes crazy? your knock sensor is picking up the misfire witch means you more that likely do have a misfire