Depending on the year of the motor There are 3 different parts under the distributor I for got what they are called one is just a rotor , an then a pickup, an then a sensor, the sensor fails on them alot.. all 3 parts where around 90.00 bucks, after reading your post and symptoms it's definitely something in the distributor ignition wise I would just replace all three parts look them up on the internet and get them from a Mercury dealer no aftermarket...Hope this helps you been there.