I have an ISE H-770-10 instant hot water heater on my sink. It's a bit old but it works, except that after a period of just a day or so, when you turn the handle for hot water, it takes a long time for any water to come out. It eventually does, and it's hot. It's not leaking out the bottom, and I'm pretty sure it's not boiling out (it's not that hot, and I don't hear it spewing, ever). What could be wrong? (Or do I just need to buy a new one... )