Trouble Shooting Tips.
Mark, to determine whether the problem is in the inlet piping or the outlet piping from the water heater, connect a garden hose to the water heater's tank drain and run it to a sink drain or outside and have someone turn the drain valve on.
Possibility 1.
If the pressure is "good" from the hose, then you know that something is either partially blocking the outlet of the heater, OR something loose got pushed into the outlet line, (hopefully not in the outlet line because it could be ANYWHERE in that line and hard to find), but could maybe be blown back out with air pressure applied through a hot water faucett if the tank outlet was disconnected from the line. If the problem is inside the heater on the outlet, then something is loose inside the heater and you might or might not be able to blow it out through the drain valve and would probably need to exchange the heater for a new one.
Possibility 2.
If the pressure is "not" good from the hose, then the problem is either in the inlet line or internally in that line inside the heater. You could disconnect the inlet line from the heater and use air pressure through the tank drain valve to try to blow the obstruction back out of the heater or inlet line. If you cannot clear the blockage, then an exchange heater is in order.
Good Luck.