Picture trimming on some resolutions

I've recently started to reinstall my PC cab and once again I came to strange problem using soft15kHz. When I install default resolutions in most cases picture is trimmed from 1 do 8 lines. Mostly about 8 on upper part of screen, sometimes 1 or 2 lines from left sides, sometimes even form right side or bottom side (on the bottom final line is partially drawn or flashing at the end but that's problemy how TV works).
To bypass this problem I've recalculated new resolutions when total lines are expanded by 16 lines horizontally and 16 lines vertically and active lines are repositioned, for example:


modeline "320x240@60" 6,964 320 350 388 438 240 242 245 265 -hsync -vsync - this one works and it's not trimmed


modeline "320x256@60" 7,411 320 350 388 438 256 258 261 282 -hsync -vsync - sadly this one will not work as it exceeds 15 kHz too much




My question is how to bypass this trimming so 320x256@60 could work and how is this trimming possible?


BTW. Effect is the same with 31kHz resolutions and Samsung 19 '' PC CRT so this is not TV problem. Overscan was removed in TV as you can see even too much (you can see 3 colour test lines on upper side). I'm using Sony 29'' flat CRT with Asus Radeon4350.

Images are from Arcade_OSD program so it's clearly visible that some lines in some resolutions are missing.
Anhang 13430Anhang 13438Anhang 13437Anhang 13436Anhang 13435Anhang 13434Anhang 13433Anhang 13432Anhang 13431Anhang 13439

EDIT. After running some PCBs (several lines cutting on Super Contra, Metal Black)I've learned it's TV either by design or TV fault. Since it appears also on CRT screen it's probably by design