Text screen uses CGA for horizontal and vertical timings, and is monochrome although CGA defines 4 or 16 colors. If I recall correctly, it's 15.7 kHz for horizontal and 60Hz for vertical. My notes are elsewhere. Check if the scan converter goes this low in frequency. Your symptoms seem to me like the scan converter is locking into the 2nd harmonic - ie, 31.4 Khz which is within the VGA range that these scan converters are usually used for.
Graphics screen uses EGA for video display, as far as horizontal & vertical timings, though it's monochrome and EGA defines 16 or 64 colors. Of course, you've already got this one sorted.