The terminals are in good shape? No sign of roughness, arcing, etc?
Usually they are pretty rough (at least in my experience).
Yes, it DID sound like not enough current to run the motor and the solenoid
is at the head of the line of "usual suspects". Do I remember that you had
tried several different starters? And that when you hooked the hot lead to
the little cable that goes inside the starter from the bottom bolt on the
solenoid that the starter seemed to run OK?

It could be that there is a bad connection inside the starter, maybe the
brushes, dirty commutator, etc. The starter field windings are so heavy they
hardly ever give trouble. But the electrical connections between them sometimes
do. One thing you have to do before you can disassemble a starter is to remove
a screw that you can see after you take off the endcap. It connects the field
to the brush rigging. It will be pretty tight. Use a good sharp screwdriver
with as thick a blade as will fit in the slot and push down hard while you are
unscrewing so you don't mess up the screw head. It's metric so don't lose it!

Hope that this is helpful,
--
George Downs Bartlesville, Heart of the USA!