I am playing with plupload 2.1.3 and can't get it to complete an upload using their custom example. I get a 103, unable to move uploaded file. If I look in the upload directory I see the file I tried to upload listed as test.png.part. My php.ini lists /tmp as my upload_tmp_dir.
From this I gather that the permissions are correct (I even set everything to 777 temporarily -- go difference).
This is a fairly newly configured server so it may not be related to plupload.
You have to check your PHP configurations on php.ini file. If your upload is constantly being interrupted you either need to increase the maximum upload size or the time before your PHP script times out. Check this variables:
post_max_size
upload_max_filesize
max_execution_time
You can either increase in your PHP configuration file or increase in your script using the ini_set function. Check the manual for more details. PHP Manual