I think the phrase "jump the shark" was specifically created for the last couple of season of The X-Files. Robert Patrick did a pretty decent job picking up the slack left behind after David Duchovny walked off the show, but it just wasn't enough. Add that to the fact that X-Files had one of the absolute WORST series finales I've ever seen.
The first X-Files movie was ok... but then the show...
"Due to constant content theft, this is only a sumary of...