I have seen improvement with a different (better quality) HDMI cable, and avoiding HDMI splitters / converters / adapters along the way. Trying a different resolution may also help.
In general however, there are screens which work poorly with the Pi, despite good cables and config_hdmi_boost. If this is the case, there's nothing you can do.
I suppose this is because your screen does not support the mode you're trying to set (hdmi_group=2, hdmi_mode=28 seems to be 288p with pixel quadrupling).
Start the Pi with the screen switched on, note the mode it is set to (using tvservice or otherwise), and set that mode in your config.txt.