Hey there, the first thing to do is slow it down. Play much slower than the actual tempo of the song, and make sure you are picking that problem string cleanly and clearly. Repeat for 5 or 10 minutes at the slow tempo, then raise the tempo just a touch. Repeat. Using this methodology, work your way back to the actual tempo of the song.

There's no magic bullet here. Playing the part correctly with plenty of repetition at slow tempos is the only way to conquer any problem areas. In short -- practice, practice, practice!

Let me know how it goes!

Mike