I suspect that the TV signal is not synced properly when switching to or from the wait routine in RAM (rotating + sign) and the text kernel.   maybe @Thomas Jentzsch or @Andrew Davie can have a look at the code (maybe) here:     https://gitlab.com/firmaplus/atari-2600-pluscart/-/blob/master/source/Atari2600ROM/PlusCart.asm#L941