Weird encoding in Media Encoder

Hi there,

I will highly appreciate if somebody can give an answer of my issue. I have a Trapcode composition in HD which contains some audio-reactive visuals and a grunge background. I have created this in 30 and 60 fps and export as PNG sequence and Quicktime MOV.

When I tried to encode it for Vimeo or with just a Custom H.264 preset I received a very weird result. The first one second of the video is fine but after that I have a mass drop in quality so everything goes pixelated. I tried to change the bitrate and level and set them to the highest values. Than I received an excellent quality in approximately 9 GB. No chance to upload anywhere.

I have the same issue on 2 different machines. Still cannot figure out the problem. I see that it is something about the bitrate but how can I get the good quality in a smaller file size.

Thank you in advance!

Anna