What year was the home built? Could it be the pipe nipples at the water heater? Sometimes those can either close off, or if you have a check valve, it can malfunction.
If the pipes are galvanized, the hot will close off sooner then the cold, but normally it's a little sporadic until it all closes down. On a repipe, I always recommend replacing the hot and cold together, or you wind up with too much hot and not enough cold to balance it down.
If it's on lav or kitchen faucets, and the pipe in the wall is galvanized, it can be the nipple that the shutoff is threaded onto. Where the brass meets the steel, it can close off. Sometimes replacing the shutoffs with new nipples is a temporary fix until the home is repiped.