Can't drag video clips onto my PC. Files transfer as partial.

Trying to drag & drop video clips from my iPhone 5 into a Windows 7 folder. The files appear to copy successfully, but then the files are partials. And the file sizes come out different every time I retry. Smaller files (less than 20MB) will eventually copy completely after several tries. Larger files (one is 250MB) have copied partial for over 50 attempts now.