340 cranks over forever when warm

the 340 in my 68 dart starts absolutly perfect when motor is cool but after driving for around 15 mins or so after motor gets nice and warm I have to crank it over for a good 15-20seconds to get it to start again. I am thinking this is a timming issue maybe. Which direction would I need to turn distributer a little to test this out? (clockwise or counter clockwise)

thanks!