Flickering horizontal lines on external monitor when connected to Thunderbolt port - 2015 Macbook Pro

Hi. I've been getting flickering colored horizontal lines on my external display when it is connected to one of the Thunderbolt ports (via Thundebolt>VGA cable) on my 2015 retina Macbook pro.

• The lines are not present when using the HDMI port to the HDMI input of my display
• The lines are present in clamshell or dual display mode.
• The lines are present in "safe" mode.
• I reset the SMC/PRAM, and this did nothing.
• The lines are not present on the 13" macbook display
• The lines get worse when there are multi-colored images on the screen.

Thanks much.