truck wont crank and all has been checked ok. HELP!!!!!

truck dies. it'll go a few days and be fine, then it dies. then it'll go a little bit and when you try to crank it, it dies. It has never died on me while driving just when i try to crank it. it isnt constant and very irregular.

I have checked all my radio wiring, all is good. Battery checked fine, Alternator checked fine, and starter and cellanoid checked fine. what else could it be?

when i drive down the road the battery gauge will fluxuate lower and lower then pop back up to regular. What the fuck. help me people. i need to get it back on the road and dragging!