Starting Issue, nothing I've read has helped

Here is the problem.
I turn the key, all the lights and radio work and I can hear one loud click and then a buzzing sound (which I think is the starter spinning but the gear is not popping out). I push start the truck (its a standard) and it starts. Or for the first few times this happened, I fiddle with a loose neg terminal and it starts. I drive for at least half an hour to an hour, turn off and it starts up again.
So,
-I replace the Neg terminal connection, does not fix the problem now.
-I test the battery terminals: 12.5 Volts so it cant be that
-I attach a portable 700amp 12.6 volt battery booster/starter and it wont start, so it cant be the amps.
-I replace the starter with a new one, still does the click, then sounds like the a buzzing again and doesn't start.

My biggest question is why does it work ok once the truck has been running for a long time? It would lead me to believe its either the battery or something non electrical...

Thoughts?

Today I will attach the portable booster again and try with the new starter, I will also volt check the starter relay, check the engine ground connection and check to see if power is getting to the starter at its connection points.

Thanks

 

Ended up being a simple fix, because I'm an idiot. I was too focused on the negative terminal and when I started inspecting the positive side I realized the connection from the battery connector to the individual wiring was pretty corroded. Once I cleaned everything up it started up no problem!
It makes sense that testing volts and attaching a battery booster wouldn't diagnose the problem because the fault was just past where those connections are made. I'm going to leave the new starter in since the old one has over 350,000 kms on it.
Hope this helps if someone else is having similar issues.

 

Right on, man. Right on!

 

Thanks for posting back with a reply. I'm sure somebody else will run into this situation and pull up this thread on a Google search.