For what it's worth, the mechanic I go to thinks the earth is under specified, so heat is precisely the problem, creating excessive resistance. He added a secondary earth cable roughly a year ago and I've not had any problems since. It is still slightly sluggish turning over but it seems to have tipped the scales so it's never not started once hot, since then. Might be hocus pocus I've no idea, but it's worked for my ride - still on the original battery on a 2014 R9T, optimate on whenever possible...