Lately when I turn on any faucet in my house the water comes rushing out at about twice the normal pressure for about half a second and then drops to normal pressure. I am worried the pressure is building up to high in the pipes. If I turn off the faucet and then turn it back on within a minute the pressure comes out normal. Its only when the water hasn't been used for a while that the pressure seems to have built up in the pipes. I tried turning the pressure down on my pressure regulator by turning the adjustment screw on top one full turn counter clockwise but the water still comes out way too high when the faucets or the clothes washer or shower is first turned on. After a second the pressure drops way down now that I adjusted the regulator.
The problem is your expansion tank, or lack thereof. Your home has a check valve somewhere in the system, possibly included with the pressure regulator or as part of the municipal water meter. When your water heater turns on, the heating of the water causes it to expand, and that pressure has nowhere to release until you open a faucet. What you need is an expansion tank installed in your water lines. If you already have one, then it has failed (they usually have a bladder separating the air from the water, and this can fail over time or if not properly maintained). Installing a tank isn't too difficult, but will require shutting off the water to your house, cutting your pipes, and adding a T to your water line. If you have an existing tank, then they are often threaded and are simple to replace (the connection is often threaded and can be screwed off). Note that if you don't fix this, valves in your toilet and other fixtures will start to fail, or the TPR valve on your hot water heater will begin to leak water.
You don't need an expansion tank -- your pressure regulator (or pressure reducing valve) needs to be replaced. Water pressure on the city side is typically higher than what the consumers (toilet, shower, sinks, etc.) in your house are designed to handle (typically 50-60 psi). When the pressure regulator starts failing it will bleed pressure from the city side into your house lines causing higher than normal water pressure due to a failing diaphragm inside the regulator. Buy a new pressure regulator, install it (super easy) and your problem will be solved. Areas with high city pressure (it's around 90 psi where I live!) typically need a new regulator every 15-20 years. If yours lasted 40, you're very fortunate!
user14440 is exactly right. Replace the regulator. I had the same situation. I never had an expansion tank for 25 Years and never had this problem. I messed with my regulator to improve my sprinkler system last summer and shortly after, I started having build up and failed toilet valves. Adjust your pressure valve lower. If the problem persists, replace it.
I used the regulator on the main line and turned it down 10 pounds, from 70 to 60 pounds and my problems stopped immediately. No more faulty toilet valves or bursts of pressure from faucets first thing in the morning. You can get a pressure gauge at any hardware store. It screws on to any hose bib or washing machine supply valve.
I have this same problem. It only started after I changed my shower valve... I suspect the valve is bleeding the hot into the cold and creating the problem.