2003 e350 5.4 running rough
 

I have a 2003 ford e350 van with a 5.4 engine and about 85000 miles on it. While driving on the highway the other day it started running rough and then backfiring a bit (it was raining). The problem worsened as I drove about 80 miles.
After a stop of 4 hours I started driving again and the problem continued. I had thought that if the problem was due to moisture that the hot engine would have dried it out. The service engine lite came on.
The following day the van ran more smoothly and the lite when out.
The next day (yesterday) I drove the vehicle home about 160 miles pulling a large camping trailer. At first I could detect some roughness but it improved somewhat as the miles passed.
Today I used a code reader on it and it showed a code of 303. I had thought the problem might be an O2 sensor due to simular experience with another ford I owned before. Any ideas? AT the moment it seems to be running OK.

Thanks,
Mike