Grid of dots on projection

Hi. I'm performing someone else's Jitter patch, and for reasons I don't understand a grid of tiny red dots appears on the projected image.

I've run the patch before without this problem, but now for the first time I'm using HDMI out rather than VGA.

Someone told me that perhaps because this is an old patch with low-res output (320 x 240!), that HDMI upsampling is causing the problem. Could that be? Would increasing resolution of my matrices fix this? Any recommendations for new resolution?

The patch runs in low light (this is part of the concept of the work), and so I also wonder whether the dots could be artifacts of low-resolution plus low light.

You might wonder why I just don't test these ideas, but I have very limited time in the performance space; and so I want to have possible solutions programmed in advance before I'm there again, so that I can pull them out and test as quickly as possible before the show begins.

Thanks for any help!