<VV> Strange battery / starter problem

[Kinzelman, Andy]

Guys,
I have a car I have been moving nightly for the past month, or so.  It does have starting problems when it's damp out - and is hard to get running cleanly on all cylinders.
Tonight I was trying to start it, and it was cranking normally.  Suddenly it would barely crank - but I have never seen  a battery drop off voltage that quickly.  After about a half hour I tried again - this time it cranked normally and started.  As I drove it around the corner it sputtered & died.  Again, after a minute or so it very suddenly would barely crank over.
Any guesses as to the problem - starter maybe?
Thanks,
Andy K.
Wisconsin