A couple of days ago I upgraded from my onboard Radeon HD 4200 to a Nvidia Geforce GT 730.

Before I was experiencing stuttered playback on large file in kdenlive which the new card seems to have solved. Unfortunately I can no longer use composite to create video in video.

When I set two videos in Kdenlive and set composite the video in the box is corrupted by the underlying video bleeding through. If the underlying video goes black or the boxed video is over a dark object on the underlying video it appears normal.

I'm using Slackware and running the 352.41 proprietary driver and using Kdenlive 0.9.8. I was unable to find anyone else having this issue doing a Google search so I came here to see if anyone has any ideas why the opacity that is set to 100% is allowing the other video to bleed through.

Any help would be greatly appreciated as I really want to continue using Kdenlive if at all possible.

Hello,
Kdenlive 0.9.x doesn't offer GPU effects, so I don't see what the video card change can affect the results.
You could also try to switch between OpenGL and SDL display outputs in settings, but again this shouldn't affect MLT composite processing.
Finally, I would recommend to use Affine transition (GPL) instead of Composite (LGPL but limited & less precise), or even frei0r.cairoblend but not sure it is available in your old install.