I've had a few files that seem to be going backwards in downloading, they get to a certain point then seem to go back to near the beginning then start again, which is highly annoying.

Tonight I had a big file that was at 86% and was going along at a good pace after weeks of inactivity, then before my eyes it went right the way back to 20% no reason

What is going on and how do I fix it...thankyou.