Next time it does that, pop off the air cleaner and look at the choke plate. I think it has an electric choke, that can malfunction. If the choke plate is closed, stick a pen in there to hold it open just until it starts. Then remove the pen, pencil, what ever, and it will run fine. If it's not the choke plate, then it's probably the mechanical advance drive in the distributor. It's telling the distributor to advance the ignition spark for a cold engine.