Hi there
I would suggest you install Netfabb studio basic. Its free. Just register and you are ok to go.
Then import all STL before you print them to check for errors.
If your file have errors inside, then you will get triangle as market on picture with 1.
https://www.dropbox.com/s/ocsr76w9jf...00.14.png?dl=0
In this case you have to correct it by clicking on 2.
then follow default steps for correction.
After that save your corrected file under Part->Export part->as ASCII
and you save it under new name.

Then you are ok.
I tested your part, as you can see from picture, it has errors inside.

Regards
Roberto