Cannondale Headshok - damping slow? broken lockout?

I have a Cannondale Headshok Ultra Fatty on a 2000 Jekyll.

On a ride yesterday it started making a loud "THUNK" when coming off a
log, or rock. It's like the rebound damping is super-slow and not
letting the fork return quickly enough. When it does pop back out, I
hear the "thunk" as the fork tops out. At least, that's what I _think_
is going on in there.

I also noticed that the lockout doesn't, anymore. And if I turn the
lockout "on" then the damping is even slower, with an even louder
thunk.

Any thoughts on what might be causing this?

It's going to the shop in a couple of hours, but I'd like to get 2nd,
3rd, etc. opinions.

Thanks all for your help.