Is the uploading/downloading on the same PC as the Skype call. If so the PC may be affecting the quality. Just recalculating a spreadsheet can lead to breakup.
You can improve the voice performance by changing the priority in task manager,
Application, right click, go to process, right click and set priority to high or real time if it will let you.
This should prevent the other things on the PC from contending with the voice. This may need doing on each call, but there is an open source app that will fix it permanently ( I think it is prio_win32_200-2667).
Same will need doing at the other end. Recommend not using wireless connections as well, this would remove another contention issue as well.