I think that the Rams had their best 1970's team in 1977. But, right before the playoffs, it rained and rained. Then they had a high school football game in the coliseum the night before the Rams' playoff game against a weak Minnesota team with a QB named Bobby Lee (Tarkenton was injured). The playing field was a quagmire for the Rams-Vikings game, and the Rams could not do anything in a game that was considered an easy win for the Rams going in.
So, the Vikings, with probably their weakest team since the 1960's, upset the Rams, and Dallas had an easy win the next week. 1977 was a year when everything came up gold for the Cowboys. In the Super Bowl, Dallas played a Denver team that was in the playoffs for the first time, and won.