I'd start off by replacing the rubber vacuum hose that runs from the manifold to the wiper motor. Putting some oil on the linkage under the dash helps. What's your vacuum at idle? It should be around 19 ish inches. If your vacuum is good, and you have no leaks, its time for a vacuum motor rebuild. You can send it to Ficken wiper service. That's what I did with mine. It works as new now. It does cost more than buying one from one of the vendors, but, he replaces the paddle and stuff. Plus, it'll last forever afterwards.
