I had the same problem and it was a mixing valve problem that showed up in the winter when the supply water was about 34F.
I took the temperature control stem out of the mixing valve.
I also found that keeping water hot in the summer was burning a lot of oil just to keep the boiler hot for hot water. I put in a 40 gallon electric water heater and piped it in series with the tankless coil, with the tankless coil outlet feeding the hot water heater.
In the summer, the boiler is off and electricity heats the water. In the winter, the boiler temperature is hot enough that the electric heater never comes on. The saving in oil cost at current $2.00+ per gallon is twice the electric cost. Since you are only one in the house, your saving would be even more.
It paid for itself in one summer for a do-it-myself installation where I was only paying the cost of the heater and a circuit. It would be more if you had to pay someone to do it.