Before you spend a DIME. PLEASE DO THIS!

The EXACT same thing happened to me with the wiper slowing down towards the middle.

All I did was lube the wiper mechanism.
[list=1][*]First, set the wiper in the straight up position by turning the key off at the right time. It took me a few tries [*]Now remove the middle cover piece by pulling down and out on it. [*]Third, Remove the main cover piece by pulling out and up on the two release tabs (circled in the picture) . I used my fingers, but you might need to use somethin pry it. [*]Before you pull the cover all the way off, set the wiper to near full extension (around 45 degrees), now you can take the cover off and slide it up just a little to expose the mechanism.[*]I used White Lithium Grease on the mechanism. It comes in an aerosol can. It is normally used for boats, so it is waterproof and when you spray it on, it sticks. Use whatever grease you'd like. Particularly grease the two vertical sliders. Also grease the top most slider. [*] Pop everything back together. Hopefully that was the problem! It saved me around $1500!!![/list=1]