Blitz:
Hard starting a fully-warmed engine in cold temps might
indicate a problem in the primary electrical circuit.  If the
starter is not defective, i would look at the battery itself,
the cables, connections, and engine grounds.
If the alternator is producing adequate amperage and the
battery is good, then look for corrosion at all cable ends.
My personal preference is to change the primary battery
cables ( hot and ground ) with welding cable in a larger
size, and soldered cable lugs.
I also like to use a spare battery and isolater system
mounted in the engine bay or under the rear seat with
the ability to switch in for starting ( can be as simple as
jumper cables ) , charges/drains independent of main
battery.
Dave