George Halas won 5 NFL Championships before he finally had back to back losing seasons, falling to 3-8-1 in 1953. Halas and the Bears rebounded with consecutive 8-4 records. He stepped down for a couple years, came back and won his 6th Championship in 1963. But Halas owned the team.
Curly Lambeau, Bill Parcells, Hank Stram, Tom Flores, and Steve Owen are the only other coaches who met the following criteria:
They all won 2 championships with the same team, and each of them had at least one year with a losing record in-between championship seasons. However, none of them were "down to the low."
Lambeau went 5-7-1 in 1933
Owen went 5-6-1 in 1936
Stram went 5-7-2 in 1963
Flores went 7-9 in 1981
Parcells went 6-9 in the 1987 strike season.
Interestingly enough, the last three guys (the only "modern era"/post 1960 coaches) all had the losing season the year immediately after they won their first title.
Marv Levy retired at age 72. He was not forced out by Ralph Wilson. Levy has said that he later regretted retiring and wanted to get back into coaching, but nobody would hire him. He did become Bills GM in the mid-2000's. He brought some class to the organization, but couldn't build a winning team.