Go-off-the-boil Definition

verb
(UK, Australia) Cease to boil when heat is no longer applied.
Wiktionary
(idiomatic, UK, Australia) To lose interest; to pall.
Wiktionary
(idiomatic, UK, Australia) To become of diminished intensity or urgency.
Wiktionary
(idiomatic, UK, Australia) To become less successful.
Wiktionary