Mac Pro showing horizontal static line when booting

I have a late 2010 Mac Pro (Mac Pro, macOS Sierra (10.12.3), Model 5,1 A1289 2010 8 Core Xeon 2x) that is showing an artifact every time it boots:

The boot starts fine, that happens, screen goes black, then it recover and boots normally. Just bought used, so not a lot of working experience, but so far this is only evidence of problem. But I worry it's a sign of a dying graphics card. It's exactly same as this MBP: https://discussions.apple.com/thread/7783901?start=0&tstart=0 which makes me wonder if it's the same kind of "Radeongate" graphics problem that plagued the MBPs from around this time.

An Apple Hardware Test showed no problems, but I gather they don't always test graphics cards well. Is my graphics card dying? If so, which one -- the one in the logic board or the other?