what kind of toilet are we talking about here?
it sounds like you are getting pressure fluctuations in the main.. install a prv and set it to 60.
the municipal water pressure will vary but will be higher that your ideal system pressure, so during the day when you are not using fixtures it will climb somewhat.
appologys I cannot think very awesomely right now.. fighting a flu.
As a service plumber I am hearing " pressure, no prv, fixtures acting weird " and I would reccomend a prv. to keep the pressure regulated.
Perhaps others can offer a more Scientific insight As I currentlly lack the Wit.