Cp4: slide shows another slide during transition 'fade'

I'm using Captivate 4, and have created a project with 22 slides. Each slide has a background consisting of a screenshot of an application. No imagelayers on top of that, except for highlights and textboxes. Also, each slide contains audio and a few slides contain Flash video.

Problem: when using the transition option 'fade' from slide 4 to slide 5 (slide 5 - transition 'fade'), another slide shines through some of the white space. The slide that shines through could be three different slides, which all contain the same image.

In other words: when transitioning from slide 4 to 5, slide X (could be three identical slides) shines through the white parts for half a second.

I have tried:

- Creating a new slide within current project, with a new background and copying the highlights etc (same problem)

- Copying all slides to a new project (same problem)

- Adding blank slides before and after the slide that shines through (still slide X shines through)

- Checking for transparency (none)

- Inserting the background as a new png and merging it into the background (same problem)

- Inserting the background as a new png and showing it to the end of the slide wthout fades (same problem)

- Removing the transition altogether (works, but I need the transition!)

I have run out of options. Can anyone help me?