you wouldn't by any chance be on well water?

Is anyone using bleach inside this tank?

Are you using imported or BIG BOX stores for replacement parts?

This is county water, and I am getting my parts from the local big box home supply. I completely replaced one commode with one of the commodes advertised as being able to flush golf balls. I thought it was the water shut-off valve, the valves we had were the type you have to turn 2 complete turns to shut off and have rubber bushings. I replaced all the valves (we have 4 bathrooms) with the new type that you only turn a quarter turn.

Is it possible the water pressure coming into the house is fluctuating and causing the commodes to over-fill?

i think you need a water softener system.

Happy Holidays!

The 1/4 valves AKA ball valves are great but not designed for throttling as a globe type

About excessive pressure I don't think so as you would hear a lot of noise when any fast closing valves are working such as a washing machine or dishwasher (Solenoid)