Is there a way to make Siri listen properly?
As far as I'm aware there is no way to get around this if you want to continue to use Siri from the lock screen or by holding the home button.

Your only solution at the minute is either to make sure you say everything without pausing or enable and use the dictation feature on the keyboard - It listens indefinitely until you tap on 'done' so you will be able to pause to think without getting cut off.