In case it's relevant: Several days ago the pilot light went out. I attributed this to having to use the shop-vac in the area and possibly blowing it out. After lighting it, it went out again after a day. I cleaned the vents, re-lit it, and it's been OK since.
However, having set the heater to the ~120 degree setting (the arrow), I noticed the water hasn't been getting very hot. Checking the thermostat again, I found it at the "tick" below the arrow (the 120 degree setting). So, I turned it back up to the 120 degree mark, it kicked on, and appeared to be working normally again.
Later that day the water was still only "warm." So, I checked the thermostat again -- found it AGAIN was lower, this time about halfway between the 120 setting and the tick below it.
Is there a technical explanation for this? Or do I need to call the ghostbusters?