My cross dissolves sometimes dip to black

We have an issue where crossdissolves applied to either video or graphics in tracks above the storyline SOMETIMES become a dip to black with a hard cut back to the storyline clip below. This does not happen always. It does happen more often when using two or more layers and crossdissolving all simulateously. The solution seems to be to delete and reapply the dissolve. Sometimes two or three times, but eventually it applies correctly.

I wonder if this isn't the result of an old FCP7 habit or some other issue. Any guidance would be appreciated. Two screen caps are posted below.