98' Grand Prix - Rough Starts

I have a '98 Pontiac Grand Prix (better known as our Grand Pile). The last few days it has been hard to start and runs rough when it does. At some point, it smoothes out and runs fine. I've scanned it and received the following codes:

P0403 - Multiple Random Cylinder Misfires P0303 - Misfires on Cylinder 3

Any clues as to the most common cause of these errors? Faulty injector or coil pack? If so, any clues as to how to zero in on the problem?

Thanks.....