i own a 1998 mercury mountaineer. Everyone has told me that thee explorer is where i'll find parts for it. So i joined this forum hopeing to get some help fixing it. It has a 5.0 engine. Its problem is that when it reaches a speed of 40mph it starts to buck. It will do this untill it reaches 45mph then it will stop. Anybody else have or had this problem. Anybody with a solution.

 

From what you describe, it sounds like you are losing spark at times.
Anything from a questionable coil to a poor connection in the ignition system could cause the loss of spark. Tests from the crank sensor to the ign module to the PCM to the coil(s) might reveal the breakdown point. Of course you could just throw parts at it, but that can get expensive. Better to test and don't guess.