Error when import questions from file located in computer

When i try import questions from a file locatted in my local computer, sometimes (9 in 10), give me this error: an error has ocurred in import process: is not possible to read import file, or file is empty.
Before i can see a window alert, saying that elements is null and refering to URL ... questions/import.php.
Sometimes the process works, and he works fine if first i upload the file into the server, and then i use it.