Thanks Funky for the additional notes (and the correction regarding the 'advj' comand
)
I FOUND THE SOLUTION !! 
Actually, it was very simple, dunno why i never thought to do this ?
The solution is that I SET A RESOLUTION IN MAME THAT IS A BIT WIDER THAN THE NATIVE GAME's RESOLUTION
Ok, but be sure the resolution modification won't affect the correct ratio of the pixels on screen.
If the ration is wrong, in galaga by example, the shape (or size) of your ship could be different on different part of the screen
...or the move of your ship would not be 'smooth', or other sprites on your screen could have missing pixels because mame removes some lines or columns
...
A good test I use to do is trying Xevious and see if there's no deformation during the vertical scrolling. If the ratio is good, you should have a perfect scrolling...
After that, just test Wonder Boy and see if you have a smooth scrolling too
Hey DN, for your Rolling thunder overscan, You should try to set 304x240 like I did for my vertical games : You'll see, the game will fit perfectly the screen !!, as mame will add 8 blank lines on both sides !
But I dunno if your Rolling Thunder cab is based an the original PCB or a PC+Advancemame configuration. Maybe If you're a purist, you may go for the PC+advancemame to replace the original PCB, and to have a fullscreen no overscanned game.
Well, as I said I'm using the real board, and it's fine for me like that... it's a "root" display, like it used to be in the past...
Anyway, there are many solutions for all our config problems... and only experience (testing a lot of different GFX card, soundcard, TV Display, ADV configuration, ... ) and informations sharing will give us all the answers
