Starter issue

I have an 87 Chevy Astro 4.3 v6
and I can't get it to crank.  
Replaced a bad starter about 3
months ago and then over a 2-day
period, there would be times it
wouldnt do anything when I turned
the key.  I was sure it was just bad
connections.  I would loosen, twist
and re-tighten battery connectors
and It would usually start right
away.  Well, it got worse and then
finally just wouldnt do anything.  
What Ive done so far is 1) checked
all connections so they are tight.
2) checked all the fuses (the first
time I checked, the ignition fuse
was blown.) I replaced it, but that
did nothing.  The battery is very
good, connections tight and I have
good ground.  
I should add that the solenoid only
engqged a few times before it
stopped.  I figured a short, but I
couldnt find anything.  So, I took
the starter out and had it tested.  
Worked fine.
I just put it back in and made sure
all connections were good and
now I can hear the solenoid
popping but the starter wont
engage.
My question(s):  Is the wire that
connects to the same post as the
battery have to do with cranking
the engine?  I ask because I really
dont know where it goes to.  I do
know i have juice to the starter.
Is there a relay Im missing?  
Would a bad ignition act like this.
I am completely dumbfounded
over this.  
I am going to run through the
fuses again to make sure I didnt
blow another, but after that, I have
no clue as to what it could be.