Strange problem, truck won't start

[GreenMystChevy]

Truck won't start! and I can't figure out why. Here's the story:

Two month old battery, get into truck, try to crank it over and it very slowly cranks and then dies, kinda like I used the last bit of juice even though I cranked for 2 seconds. Start up another car and run jumpers to mine. Wait a few minutes and fire it up and let it warm up while I go inside the house. I hear the truck die from bad carb adjustment, go outside to start it back up and, click click. Nothing, headlights turn on, heater fan works, no starter turning at all.

Throw a different batter in and drive for 20 minutes turning it off here and there and starting up just fine. Got the battery tested and it's good to go, just needed to be recharged. Get to school, park it, try to restart to park in a different spot and nothing. Come back after class a few hours later, wallah, starts up like nothing happened.

Again, this morning, a few days later, my battery was dead when I jumped in. Threw the other battery in and off I went. On my way to school I shut it off while driving on a back road and threw it into neutral and tried to start it. Nothing at all so I just compression started it and kept going. The volt gauge didn't move, everything just looked normal.

Loose ground? Where should I check? Bad starter? I don't have headers, just ram horns. Both batteries are brand new. Thanks guys, I just hate being stranded everywhere, it's embarrasing!