Shimano 600 FD with DT shifters on my Centurion. When I'm in the big chain ring after a minute or two I start to get chain rub. I can trim the shifter and then I can pedal smooth with no noise but after another minute or two I need to trim it again. So basically seems like the lever is moving on it's own/not staying in one place. Is my cable tension too tight? Is this a limit adjustment? Is this just nature of the beast with friction shifters?