Either your monitor doesn’t tolerate the sync jitter. Or it doesn’t like the linedoubled 480p signal. For the latter you can try the other linemodes, for the former you would need the de-jitter mod. Or finally, it could be a cable issue.
It would help if you could try with another monitor.
The v.freq given by ossc is slightly low (should be above 60hz, not below), you can try cycling inputs and see if it will re-lock at correct refresh rate.