OK, so here is a (comprehensive?) list of TV spin-offs. At least this will help maybe narrow things down a bit? If you go through and eliminate everything but situational comedies in the late 70s, it breaks down a lot.
TV Spin-Offs
Its funny - George Clooney was in The Facts of Life (a spinoff sitcom which started in 1979, ran for 9 years), and then ER. Hmmmm...he did have quite a slump between jobs. Don't think its necessarily him though.
Jeffrey Tambor was on The Ropers, and then later was on The Larry Sanders Show.