Diverter handle getting hard to turn

I have a Grohe Classic three-handle tub/shower kit (installed in '97 - some photos are posted here), and over the past few weeks the diverter handle has been getting tighter and more difficult to turn. After turning it back & forth a couple dozen times it seems to get a little easier, but just a few months ago it turned very smoothly and easily. I tried removing the lever (assuming it comes off just by pulling, like the hot/cold handles do) but it doesn't budge.

Any ideas what the problem might be, and/or advice on how to troubleshoot and solve it?

Thanks!!

Call Grohe...most of their valves have a lifetime warranty.

Thanks. I just shot them an email. We'll see what they say Monday. I also just noticed that I've got a new water stain on the ceiling directly below the shower (in the garage), so this problem is now causing a leak. Gotta solve it quickly.