79 GMC - weather and starting issues

[79 sierra grande]

 

my 79 is pretty picky when it comes to cold or humid weather. on normal days, it will start no problem. if it doesnt start within 4 or 5 revolutions, it's not gonna. it's a very good running truck when it wants to be. but i started having this problem in mississippi in the summer time on humid days. it will crank and crank and not start. it gets no spark at all. the first time, i thought it was the control module. i had a spare, so i replaced it. 20 minutes, good to go. the second time about two months later, i didnt have a spare, so i just took the distributor cap off, looked at stuff and put it back on. fired right up. this got me wonderin if it was even the control module the first time. now in tennessee, it's pretty cold. i tried starting it today with the temp around 35 or 40. same thing. crank and crank and crank, not even a pop. it ran perfect last week when i took it out. so what gives? it doesnt cut out when im driving like it's a loose wire or something. i read an article in four wheeler about how sometimes bad body to frame grounds can short out your system or ruin a control module. where exactly are these grounds they are speaking of? also, it said in the article that an alternator sending excessive voltage or a weak diode sending unrectified AC voltage to the module will destroy it. i have a voltmeter in the truck, and it never goes above 14. so i dont think it's the alternator. any other suggestions? thanks for the help.