: Mike,
: What has worked in my home, is to provide an expansion tank at the cold water inlet to the water heater. I would be thinking a ST-12 from Amtrol, a 4.4 gal expansion tank that can help buffer the ups and downs of the pressure.
: You can, in addition, change the 1/2" pipe to 3/4" as far as you can go, however, I would try the expansion tank first. Let me know if it works.
: Terry

I disagree with Terry. The easiest thing to do is install a pressure balanced shower valve. This will absolutely eliminate the problem. -Aros