I think there is a check valve on the inlet of the pump to prevent back flow. If that valve leaks slightly, the pump will come on to build pressure again.
On some trips, mine runs the 2 seconds every 30 minutes, and then sometimes it works great without cycling. I think some trash in that valve could cause the cycling. When it's doing the cycling thing, we just turn off the pump at bedtime.
Your lack of sufficient flow in the shower leads me to believe it may be time for a new pump, but of course I'm guessing.
Let us know what you find.
Fred