both trucks acting up-same problem-wtf

[67cst]

has anyone ever heard of this with these 67-72s? both trucks run good but once and a while i jump in either the 67 or 71 and turn the key and nothing. its like the battery is completely drained. but it ran fine and started fine 10 minutes before type thing, friday night i was getting gas in the 67 and went to leave the gas station and i turned the key and nothing. i pushed it into a parking stall and got a buddy to come give me a jump, we went to his work and tested the battery/alt/starter and they all checked out fine, same with the 71 gmc, about a month ago it did the same thing, wouldnt start so i just went into the mall and killed time went back out half an hour later and it fired right up...what gives? anyone have any ideas? both trucks have hei too if that makes a difference

thanks, Derek