Bad coil. I have a 2003 F150 and just had one go bad on me. Same thing happened 2 years ago. Horrible stuttering over 45 or so. Depending on the year/model, the coils are kind of hard to get to, so you might not have a choice but to take it to someone. I went ahead and got all new spark plugs as well because bad plugs can be the cause of the coil going out.