If a team goes off the boil it means they are not playing as well as they were playing before - their intensity has dropped. Imagine that boiling point is playing at a high level then off the boil would be a lower level. Teams cannot always play at top pace or high intensity and so if a team goes off the boil it means they are not playing at the same level as previously.

This phrase can be used to describe a team's performance in a game or over a part of the season - Spurs went off the boil after Christmas and dropped from first to seventh in the league.