after 8 min of good job, suddenly "next subs" stack with appropriate ones.
The lag seems to stay the same all the way down to the end of the video (total running time ~13min)

Aegi warned me that subs res doesn't match with vid res but it still displays subs correctly while applying no changes.
vid res: 720*576
sub res: 1920*1080

I gave it several shots, setting subs to video res, resampling script. That didn't fix the problem.
Problem only seems to occur when hardsubbing with Handrake.