New to mac, Quicktime issues

hey guys, i recently picked up a macbook and i've been having some issues with quicktime. I download videos that are say, 70 mb's but quick time will rarely play them for more then one minute. this wasn't happening before but recently it's started. it's possibe to watch more thne the first minute of the video, but i have to manuely drag the cursor back to the two second mark once you finish the first minute. does anyone else have this problem or know how to fix it? is it something wrong with quick time, or the compression of the files i'm viewing?
also wondering, could it be the lack of the mpg-2 capabilities of quicktime? i find it odd that this is not a standard quicktime feature....