keynote video out scrambled

I have a 2013 MacBook Air. For the past year, for the first 30 minutes of being connected by thunderbolt-to-hdmi (or VGA) cord into a video projector onto a screen, the image on the screen from a Keynote slide in play mode, after being on the same slide for 30 seconds, converts on the projected screen to be digitized (images and words not remotely discernable—it’s not just the slide blurred, it’s just lots of pixilation). My MacBook Air screen looks perfect while this is happening; it’s just that the projected image is pixilated/digitized/scrambled. If I hit escape and then play the slide again, the normal image/slide appears on the projected screen again. If I advance to another slide (or play an automatically progressing file), there is no problem. It’s just staying on one slide for over 30 seconds that does this. If I’m not in play mode, my desktop/screen image is projected perfectly forever. It’s just when I’m in play/display mode and sticking on one slide too long that results in this. Also, it seems that after I’ve been all hooked up for 30 minutes, it no longer has the problem. Any thoughts? Thanks, in advance!