I found the problem. It would take 3 pages to explain what the problem actually was, but is had nothing to do with the drivers or the card. This must have been an issue all along. I found a section in the code where, under certain conditions, data that was alread read from the buffer would be discarded instead of being sent to the graph/disk. I fixed this little loophole and the problem dissappeared.
Thanks anyway. :O)