Tom,
Assuming your burner and stir pump are continuing to run, it sounds like the mixing valve may be going bad. I had similar symptoms on an 01 Contessa and had to replace the valve. It's purpose is to mix cold water with the constant temperature hot water supplied by the heating chamber. When it starts to malfunction it will add varying amounts of cold water. Not sure which HH model you have but on the unit I had (think it was a model 200) the valve was accessed on the rear of the unit. Mine had a temperature control adjust that I turned back and forth before returning it to its original position and that gained me about 6 months of operation before it failed and I had to replace it.
Steve