I've never been a fan of the Rain-X coating but tried the Rain-X wipers recently and have been very impressed with them. They've worked very well in the heavy Florida deluge on the VX and two of our Mercedes diesels.
I have heard of "chattering" issues from the Rain-X coating (one reason I've stayed clear or it) so maybe try removing it and see what happens. It's easy enough to re-apply if not the issue.