I'm trying to slide from one clip to another. In the preview it usually looks fine (sometimes glitches, but I always chalk it up to not having the fastest possible computer), but whenever I render the section, a single frame from the end of the transition always starts it, so there's a jarring flash of a frame ruining an otherwise smooth transition

Any ideas for how to prevent this? Thanks!

I have the exact same problem.

At first I thought it was only dissolve doing this, but then I saw all the transitions doing this.

Sometimes it doesn't do this with short/small MPG video clips. It also doesn't do this with still images.
Try saving your clips in a small Mpg format and see if that works.

I even see it when I am flipping over from one still image to the next in the preview when clicking on the frames, frame for frame.

The "effect" of the transition works fine though. The dissolve does the "fading" well, but it is the movements on the clip that are laggy in the transition. It is like this according to the frames in the preview AND in the render afterward.

Now every time I want to cross over with a fade to the next clip, the two clips look like they are lagging. After the fade though, everything is fine again. Then the clips run smovely, until of course, I need them to fade over to another clip with moving visuals inside. It doesn't look like a fine finished gradual fade.

Does anyone know what to do about this?