Rendering Problems

I have been rendering the scenes as I go along and make them, on a seperate computer. When they are finished, the avi. file is not only laggy, but sometimes cuts out frames that I have definitely rendered. I have re-checked the renderer settings several times, changed minimal things, got help from peers and still the files aren't working properly. I am not the only one with this problem but as the end of semester grows nearer it is worrying that I may run out of time. So if anyone else who may read this has any suggestions please let me know.

Edit: Found out that running the files off the desktop is necessary due to the large file size. Even when importing into Premiere this needs to be done. However, have adjusted the render from mental ray to the standard scanline, as the files that I have rendered this way in HDTV 1280x720 have been less likely to be corrupt, as even some renders in the mental ray renderer have been corrupting.