Identifying source of audio drift between camcorder and recording PC
What you are experiencing is indeed clock drift between your camcorder and your PC. Whatever their quality, two independent clocks will drift eventually. If the drift ratio is small enough and your takes are short, it will not be noticeable.

Common way to solve this issue is to use a common clock on both devices, which requires a ref input on the camcorder and a ref or wordclock input on the sound recorder, as well as a master clock generator.

In your situation, I would suggest either 


to edit at appropriate points to get back in sync. As your audio is shorter than video, it means finding places where you can cut a few video frames without it being noticeable. Not easy if the speaker is on frame all the time but it can be done if you use some other shots


or 


find out why the audio recorded on the camcorder is not good enough
for your use. Camcorders audio inputs can be tricky and require an
appropriate adaptation of the audio signal in order to get best
results.