Verify that the
raw water is not blocked and that the raw water
passage in the discharge
elbow and the 90 degree fitting connected to it (if you have a U bend) is cleaned out.
The water passages on the discharge
elbow get limed up with "stuff" as the water hits the hot
exhaust. I use a bike spoke to clean out the passages.
Could also be the thermostat or the raw water impeller. . But I'm betting its the discharge elbow as your symptoms tend to lend towards that.