The pressure release valve on my water heater runs almost constantly. The only time it isn't open is if I am using the hot water, and for a few minutes afterwards. The tank is only a year and a half old, as is the expansion tank. Ive turned the temperature, and checked my water pressure (it's somewhere between 60 and 70psi) and the new valve said it was rated for up to 150psi and 200 degrees. Not sure what else I can do to stop this from happening. Please help!!

About a minute after turning on the hot water in bathroom sink you hear a thump or what i think is a vacuum shutting a valve and the hot water pressure drops about 95 percent. if you turn the valve off and then back on, the pressure is normal for another minute or so. I have pulled the line from the shut off valve under the sink and have full pressure at that point. I have replaced the valve stem on the hot and cold side, though the problem appears to be on the hot side only. This morning my wife also noticed that we had pressure loss in the shower, but not to the point of the hot water shutting off like at the sink. any help would be greatly appreciated.