It sounds like you have enough flow to activate the pump when it doesn't start so it's possibly a faulty printed circuit board or a capacitor issue, it all depends what's happening at the pump when it doesn't start.
You will need to be within a few inches of the pump and get someone to turn on the shower. If the pump doesn't start, does it emit a low humming noise or is it silent? If it's silent it will be a printed circuit board, if it hums it will be the capacitor.
You can also do this with a magnet against one of the reed switches but you will have to do it a number of times to be sure, as the pump might start fine if it has just been run.
The printed circuit board and capacitor are available as part of a service kit but if the pump is 10 years old we would recommend replacing it.