NES game
I played it (on a famicom; came on that little yellow catridge) when I was a kid and it started haunting me to find it last weeks, but I can't really remember the name, nor much of the gameplay.
Here's what I know:

it's science fiction, it starts in a ruined city or something like that...dealing in future/space but with many closed aereas in which you fly in

it's a horizontal side scroller shooter, but it mixes horizontal scrolling with vertical, often in the same stages (but you stil shoot in horizontal direction though - this is quite uncommon in NESgames so I think it might help)
you fly from left to right and you can also move up and down (also left and right) and shoot in both left and righ directions....

it had good graphics for a NES game and was more in a serious manner - music and gaming wise, not flowerly like twinbees; also had a story and an ending so it's not magmaxm: way better, but they do have similarities

it's also not abadox, but the games wear close resemblance: the one I'm talking about has a bit more advaced graphics, and when side scrolling changes to vertical it's usually UP, not down....there's a stage where you pass thoprugh a tube (like an escalator into space), and it accelerates more and more till it looks like the background's melting

the character was humanoid, and it even might had the ability to transform into a space ship, but I can't be sure about that...


I'd be grateful for any help, I'm really desperate to find this game....

It sounds like so many games that it's hard to narrow down.

Take a look through here:
http://www.consoleclassix.com/titleslist.html?list=CAT_ACTION_SIDE_SCROLLING_SHOOTER

Clicking on the game will pop up a screenshot, see if anything looks familiar.

M.

Most likely this is S.C.A.T.