REF: Sony Vegas Pro 13

I record my video gameplay for long periods (up to 2 hours). When I edit on my Main PC there is no audio sync issues at all, but I don't want to edit on my Main PC, I want to edit on my second PC. So I transfer the gameplay through an external HDD in a USB slot. 
But when I open up the gameplay on Sony Vegas on my seconds PC, the audio is out of sync by about 5 seconds. Now this wouldn't be an issue because I know how to resync audio. But when I resync the audio to the video gameplay, around 10 minutes later on in the gameplay, it's out of sync again which makes it very impractical. 
Do you have any idea why this is happening? Editing is my full time job and having to resync audio every so often is insane. Thank you!