Tankless oil fired hot water boiler takes a long time for water to warm as soon as air temps drop Why?

As soon as the outside temp drops (40's) it takes a while for the water to warm in the shower. I have replaced the valve behind the knob but it hasn't helped. This has been going on for years and seems to be geting worse. The system is about 15 years old and maintained every year. The tempeture control valve has also been replaced. Keep in mind the ground temps are not cool enough to effect the water comming into the house. Any suggestions where to go from here? Thanks