If you didn't develop the film yourself, it makes it very difficult to know what's happening w/ the negs. Do you have some more scans you could post? That neg you show here has both white and dark spots on it, but due to the nature of the image, it's not easy to know what's up. The fact that the negs are brownish could be a lead, but give us some different scans if possible.
Even if things are in a print file, dust or moisture could be an issue, especially if high summer temps are relevant to where you store the negs.