If the ignition timing is advanced too much,it can start OK when cold,but be harder to crank after it warms up fully too...if you hear any spark knock or pinging under a load while accelerating it could be that...wont hurt to check it...sometimes when a starter is getting weak it'll crank a cold engine over OK,but when it gets heated up it gets harder to crank,and will give troubles,ditto for a battery thats losing its "beef" when it gets old,they fail under heavy load .