1975 Chevy Corvette wont start sometimes

I have a 75 vette that has a 327 in it alot of aftermarket parts. There are times when it will sit for a month and start up, I pull it out of garage and wash it and then nothing at the key(lights on dash are on) some times its opposit it wont start in garage and will after washingThen there are times when ever it feels like it it will start or not start. I have changed ignition switch on steering column, new starter and selenoid have 12 volts at starter not sure about when I am on key at selenoid, re grounded battery. The kicker is that if it dont start I hook up a batt. Charger or a jumper box and it will start every time ! This problem supposedly started when the previous owner left the car sit out over the winter, and I bought it with the problem thinking I could fix it. But. Im stumped. The car has a MSD ignition system on it