I have tried updating avisynth/x2.264/mp4box/and even tried connecting it in mkvtoolnix and nothing still it's getting async. It's first time while i'm experiencing something like this ,though earlier i used quant and now i use bitrate but it's pretty high already (2400) and doesn't seem to be cause of it.
I just to make sure made an xvid and it's fine heh.
And yeah my pc can handle 720p mp4 smoothly

my script looks like
x264.exe --bitrate 2400 --pass 1 --bframes 5 --ref 12 --subme 6 -o nazwa_pliku.mp4 o.avs
also it's my first time using lagarith with yv12 while making it but dunno if that might be case.... if it is it sux .xd