a key sign that it wont complete the download is that the size of the download starts going beserk and jumpes into the thousands and fluctuates constantly
Sorry for the delay!
That sounds like the site is using tricky time codes which may cause a problem for Jaksta. Are you able to send us a link to a specific video which causes this problem?
The next time this happens, can you send us the log file? You can find it by going to the Preferences and clicking Find Log File.
I am having severe issues while using Jaksta and www.wattsatelier.com, i have used it before with no issues and now after updating to osx 10.11 el capitan, half the time i get an err message half way through the downloads, very frustrating