I've been experimenting and this is what I've learnt. Watching the Vegas Pro tutorials, we're always encouraged to Match the video properties of the original - I did that at I got 59.940 FPS. I rendered the video to those exact settings at 8Mbps (note I recorded at 8Mbps) as well. However as you can see from the first link below, there seems to be stuttering or even frames missed.
I then changed the video properties to 25 FPS and rendered it at that, there was a performance improvement immediately on the videos. However there was still some frame-rate issues. Video below
Yesterday I uploaded a video of the dashboard at 59.940 FPS at 8Mbps and despite just flicking through the menus', I got frame-rate drops again.
Today I uploaded the same video but rendered at 25 FPS. See below
I don't know what the exact cause of this is because it is far too consistent. Is it the Hauppauge settings? I use the default and create .m2ts files and record at around 8mbps. Is it my CPU? Is it my Vegas settings? I don't know but watching videos from others I don't see them have the same problem so clearly there is a wrong on my end.