too cold just push gas petal when it starts to die
Check for fuel pressure and spark on the no start days.
A convex lens would focus sunlight to the point that it could start a fire.
It may have the wrong fluid in it.
if it is a new ford focus then no, alot of new cars have regulators on them.
COLD
weather and climate changes cause difference in size of crystals. or erosion
convex
possibly the timing belt is broken. thats not a good thing. however, there are endless possibilities like frozen fuel lines etc...
Probably not, although it could lead to other problems which would cause your car not to start.
Many things cause weather. I suppose you could say the most important ingredient would be solar power. Without the Sun there could be no weather.
engine
The most likely cause of weathering would be from windblown abrasive particles.