Daniel,
I have experienced this also, and I suspect it may be a manufacturing defect. (Mostly because I don't like to blame myself for anything).
I believe the spots on my prints were not visible at all until the paper was dry, because I'm sure I would have noticed them otherwise. They ruined some otherwise great looking prints. They look a bit like there was a bubble on the surface of the paper when it went into the fix or toner, but the sheets were agitated and I don't see any possibility that they were caused by bubbles.
At the moment, I don't have a solution, so I'd like to hear what anyone else has discovered about this issue also.
---Michael