I'm confused by what type of system you are describing but the problem is the same. You are using the same heater to heat your domestic hot water and house. When you are heating the house and the domestic hot water you do not have enough btu's. You need to have a priority relay or something similar that when you are using domestic hot water you are not allowed to heat the house. This may cause a slight dip in room temp but that is better than a cold shower.
A quick fix to try tomorrow is to turn down the thermostat before you get in the shower. This means the heater will not try to heat the house while showering. A programmable tstat makes the problem worse because some people will set the tstat to raise the temp at 6:00 and then they get in the shower at 6:15.

Look on your aquastat and tell us the settings of the high and low limit and the differential. You are probably going to find 160 low, 180 high, and 10 differential. Don't touch, just look.

When the boiler water temperature goes below the low limit setting, minus the differential (150º) your circulator should stop running. Listen for that. If not, the aquastat or the probe is bad.

When the shower water gets cold, all all the other faucets the same in the house?

When the shower water gets cold, what does the temperature gauge say?

If the boiler is being flooded with cold water from lowering the tstat at night, you may have to divert more of the water through the bypass loop and back into the radiators and never let it get into the boiler. You should have a bypass loop already installed.

I don't suspect a limed up or muddy coil, because it worked fine in the summer.

Thank you for your help.

Funnycide, I set my thermostats(digital, programmable) to start heating the house after we shower. That makes sense to me and I hope that is all the problem is.

Baymee, It is set 170 low, 190 high and 15 differential. How do I tell when my boiler temp goes below? should I just sit and watch it while my wife showers?

I have never checked the other faucets temps while running shower. In the summer running hot water from the faucet did not cool the shower water at all.

It sounds like an aquastat problem or a return line balancing problem. I would turn the low limit down to 160 and high to 180.

Yes, watch the gauge and listen for the circulator to shut off when the temperature gauge reaches 155º or so, while the shower is running. Report back.

I'm assuming you mean a tankless coil in an oil fired hot water boiler. Find the pressure and temperature guage on the boiler. Its highly unlikely your problem is with the aqua stat settings though but while someone is using the hot water keep on eye on the temperature guage and see if the burner cycles on and off within the range mentioned in an earlier post. If it does, the problem isn't with the controls. Without actually being there to diagnose it my initial thought would be your mixing valve, which is code on all tankless coil arrangements, is N/G and just letting cold water through. The mixing valve is a thin valve about 6 inches long with a small knob or set screw on top and has three ports with copper pipes attached. One port is labeled "hot", another "cold" and the 3rd "mix". With the shower running the pipe on the mix port should be about 120-130 degrees. If you have that kind of arrangement try that test and get back to us.

Good point about the mixing valve. I wasn't thinking in that direction because I've never seen one on a tankless coil.

You guys are great, thanks for all this info! I will report back tomorrow.

Well, I couldnt hear over the sound of the furnace to tell if the circulator was working. It is about 5degrees here so the furnace was running before the shower started just to keep the house at 60.
The mix pipe coming off the valve was luke warm to the touch...not very warm just mildly. I felt the two other pipes coming off the valve and both were cold. Only the pipe coming from the furnace was hot.

thanks again.

I don't think the problem is related to the domestic mixing valve. You said the shower water was fine in the summer and fall. When the heater is not heating the house how hot is the just the hot water at any faucet? If it is not terribly hot then maybe the mixing valve could be adjusted a little.
Were you running hot water when you felt the 3 pipes at the mixing valve?
I think the boiler is not turning the circulator off when the boiler water temp gets too low.
Post a pic of the boiler.