Vertical lines that appear and disappear on Samsung LN46C650

I'm helping a friend with a 15 yr old Samsung, nice TV (LN46C650) that looks clean and crisp, but when it's been running 30-60 minutes or more will show vertical lines that come and go. Sometimes the lines stay there permanently until the TV is turned off & back on. That might work for an hour, or lines might return quickly.

I removed the back panel, inspected for any visibly bad caps - not seeing anything.

Next I removed and reseated all ribbons and connected wiring to the boards. For a week or 2, didn't see any vertical lines, but now they're back.

T-con board?