are you running the out into any kind of preamp > PC? or you just running rca's > mini into your computer. check to see if your using the line, or mic input, cause if your using the mic input you have the option of i think a 20 db boost, i wouldn't use that really, cause that will introduce more noise. The only thing i can think of is that you are either louder on that song. and then get softer, or you turn your levels down on the board, and the next song is just softer. like when i record my shows i set it and forget it, but i do run a matrix so that a little different, but all i can say is test it at home, figure out how to get even levels the whole time, and then do that.. good luck.
matt