Since the weather turned cold I need to jump start my 1990 RR Classic every morning. On the coldest days it's even impervious to a jump, except after several tries.

What didn't work so far:

Installed a brand new battery
Checked battery connections (ok)
Had mechanic check for electrical drains (none)
Had mechanic check starter (ok)

Why is this happening and what can I do? I'm having a baby in a few weeks and I'm not psyched about jump starting it on the way to the hospital while in labor...