Take the nipples out of the heater and be prepaired to change them and the unions.
If there are no unions, put two on.
Could be a nipple
Could be that the dip tube is deteriorating and sending flakes of plastic into the hot side of valves or faucets.
If the tube is bad, then replace it.
I know the electric and gas co's tell you to drain often but I've not seen good results. Uaually more harm than good comes of it. But thats just one mans opinion..........