Honda Accord runs rough when starting

I recently bought a 1999 Honda Accord EX with a 4 cylinder engine and 98,000 miles. About 3 days after buying the car the check engine light came on - I took the car to the Honda dealer and they replaced the catalytic converter at the tune of $1,100.00. The car idles and runs roughly for about 10-15 mins after starting the car the first time in the morning. After the engine has warmed up it runs smoothly, and runs smoothly when I restart it during the day. I would appreciate any help you could give.