Sony Vegas Pro 13 render problems

[mekabra]

Greetings,
Recently I started to convert from analogical videos to digital (VHS). Now when I want to re-render those videos because some of them have long endings with just a noise etc.
As I started rendering (base videos was in *.avi, I render in Internet MP4 480p). I get pixelazations and blurs as in picture shown below.

I tried switching bitrates to constant, but it didn't work.
I'll apreciate any help