I downloaded a movie (fully downloaded) but when I go to play it, it buffers. What can I do?

I have a fully up to date iPad Air 2

i rented a he movie yesterday and it downloaded overnight.

i Am trying to play it but it only plays for about a minute before pausing itself, exactly like streaming and buffering but without the buffering timer popping up. If I press play again instantly it will play for about 1/10th of a second, if I wait a few seconds it will play a second or two, and if I wait longer it will play more up to a max of about 60-90 seconds. Like I say the effects are exactly like live streaming with poor wifi (which I have) and buffering.

i am concerned obviously as I a) can't enjoy the movie this way and b) the clock is ticking on the time I have to watch it before it auto self destructs

please help

I Also can't work out how to start my own thread