"Battery terminals are very clean. However, I will check the ground at the frame."

When you say "very clean", does that mean the terminal has been scraped and/or wire brushed on the inside surface? And the outer surface of the battery post cleaned the same way?

I ask, because failing to crank, but responding easily to jumper cables is often due to corrosion at the post-to-terminal contact. Jumper cables get a nice, fresh bite on the outside of the + terminal and current goes straight to the starter — bypassing the potential problem.

This would also explain a failure to keep the battery charged, since the Alternator current must "enter" the battery thru this same terminal/post connection, where the conductivity can vary with temperature & humidity.

Once warmed up, it may allow partial battery charging, but fail to pass starter current on a cold morning.

"Checking the ground" (removing, scraping, brushing) is also worthwhile.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.