Intermittent starting problem

[dzbowtie]

Hello all,

I've been having an intermittent starting problem with my truck. When it happens, I turn the key on, my red brake light turns on, but nothing happens (like a dead battery). However, battery is good and it doesn't always happen. The only relays I have is for the headlights.
The curious thing is I can attach jumper cables, or battery charger and it'll fire right up and never give me a problem. I dont even have to allow time to charge. Its almost like I just need a little extra power for the starting. Alternator works also.
Could this be a starter issue? I'm just lost as to why this happens sometimes and not consistently. I appreciate any help guys. I hate to carry a portable booster lol.
This is a 70 c10 with 350/th350.