I would guess the choke assembly may need service/adjusting. This may also lead to carb sync'ng. When it's cold the fuel mixture is usually richer as it warms up and the choke is released the fuel mixture is decreased. It the air/fuel mixture is wacked you could experience this type of problem. I'm sure someone here with more experience will confirm or correct me, so just hold on a little while.
actually, i had the same problem with my suzuki. it was a bad starter. symptoms were:
Start ok when cold (maybe slightly slow turning over)
Turn over slower and slower as the bike warmed up.
Finally, it wouldn't turn over at all. Would feel like the battery was completely dead but the lights came on.
I'd let the bike cool down and voila, it would start.