Possible Clues:

• Boiled gently
• Didn't quite boil
• Immersed, cooked and gently boiled
• Was close to an outburst of emotion
• Kept just below boiling point
• Kept on the down-low?
• Kept just below a boil
• Computer game, uncomplicated: daughter was about to lose it?
• Bubbled on a burner
• Boiled with rage?