WP typically rises in the middle of the night for two reasons: they're refilling the towers and need the extra pressure to get it there and there are far fewer people using water then. So, your 'nominal' 80psi could be a lot higher at some points in the night. Plus, depending on the city where you are, you might still have a closed system (a checkvalve either in or near the meter or who knows where) and need an expansion tank. I've heard that many places now require a ET, regardless of whether you have a closed system, as most places are installing them as their meters get replaced over time. Excessive pressure will create significant ridges as the seal gets excessively compressed.
A new cap