Video issues and alternate cards

Hi there guys,
First post and its a bit of a boring/long one.

i service a clients studio locally and we have been having issues with some larger video files.
Namely the playback is slow/jittery and tears in places. Tried every format under the sun and plenty of top end encoding programs to no joy.

I’ve been reading about the Black Magic intensity cards for some time now and they seem like a logical way forward as the issue with the playback seems to be down to quick time (little odd playing it in the external play outside cubase you have the same issues but playing it in VLC or windows media player and it s full quality with no problems)

The PC in question is beyond top spec (its abit ridiculous actually) and it certainly is not the GFX card as we have swapped this out as well.

just wondering if anyone had any experience with either the black magic cards or alternatives?

Opinions are welcome on this. thanks very much!