Switching back to indexed downtube shifters solved my ghost shifting problems. With friction shifting I'd get ghost shifting when standing to climb or sprint. Part of the problem was the mediocre friction mode on my bike's Suntour Accushift lever. It's fine in index mode.
No problems with Shimano bar-end shifters in friction mode on my other steel bike with flexy frame. The shifters have detents to hold the desired position.