I have a 14 year old home with a dedicated hot water recirculation system using a plumbed return line. Recently I began to notice tht the cold water would run cold then warm up to hot and then after 30 seconds cool off again. This happened on two handle lavs as well as single handle showeer lines. Even the toilet water chamber will be warm on the occasion when I had to clean sediment from the tank valve. I had the water heater replaced a week ago along with a new pump that is on a timer. Thought this might help but no. Another plumber suggested that the cartridge on the showers may be bad and allowing cross contamination of the cold with hot water. I'm not sure this makes sense but I have the new cartridges on the way and I will change them out as it has been 14 years. There is a check valve on the pump sytem but the recirc line empties into the cold supply of the HW tank after the check valve and I wonder whether the hot recirc water is getting diverted into the cold feed. Hard to imagine as it is a long way back up the cold to the lavs and toilets and showers throughout the house.