Crackedback,
You forgot that the starter could be the issue.....or a batt down to 12.2 v. Starters have copper windings, the resistance of which increases with HEAT......which reduces the current in the windings, reducing the HP that the starter can generate to crank the engine. That is why a GOOD battery is reqd so that the voltage drop internally IN the battery is minimised & as much voltage as possible is supplied to the starter. With low bat voltage, you start diagnosis with a good battery, not battery cables.
Unlikely to be a ground issue be cause it starts ok cold with low temp, thick oil & cold weather starts really test the starting system. Ask someone who flattened the batt trying to start the engine on a cold winters day....