Name of this game?

[robertbn]

I still think you need to check out Terra Force, as it matches your description (and you've not dismissed it as far as I can tell):

Have you actually played the game, or did you just see the demo play? If it was just the attract mode, it's possible that you never saw the vert stages.

- The game is a Horizontal Flying game with a spaceship (not robots or other thing that shoot);

yup, every other stage at least (I'm assuming you mean the player and not the enemies)

- It is from late 80's (from 1986 on) and/or at most from 1990;

yup

- I saw it in an arcade;

where else would it be

- There are static letters as power ups (the power ups are literally letters and not objects with letters in it);

yup

- To get a power up you have to destroy some enemies that come in sequence in a clockwise (or conterclockwise - I don´t remember) movement;

yup

- There are at least one stage in a cave, but I'm almost sure that all stages are in caves;

looks like it, except the vert stages

- There are not dinossaurs nor buildings in it;

yup

- The shield is the part I best remember. It is like a blue/white, "D" shaped glow to ship (there are no shield at the rear part of the ship) and it lasts until you get some hits (it has no timer). I'm pretty sure that the shield is provided by the "P" power up letter, not by the "S".

Yup, P power up and shaped like a D over the front of the ship

Yeah, man you're right. Unless there is a clone of Terra Force with horizontal stages only, Terra Force is the game. I really didn't remember of those vertical stages, but the others are exactly what I have in my memories, so I think I (you) found what I'm looking for.

Thank you so much for the help.

[ExedExes]

Glad that's over. It only took us 6 pages to ID a horizontal shmup. We need to step up our game next time

[nightrnr]

Yeah, man you're right. Unless there is a clone of Terra Force with horizontal stages only, Terra Force is the game. I really didn't remember of those vertical stages, but the others are exactly what I have in my memories, so I think I (you) found what I'm looking for.

Thank you so much for the help.

Glad I could help.

Time does weird things to memories sometimes, in my experience anyways.

For example, I just had an arcade ID-ed for me in another thread that had a steering wheel and a light gun, neither of which I really remembered. And had an electrified slot in front of your vehicle, when I initially remembered a claw.
It's weird what gets blocked out or altered over time.

[ExedExes]

For example, I just had an arcade ID-ed for me in another thread that had a steering wheel and a light gun, neither of which I really remembered. And had an electrified slot in front of your vehicle, when I initially remembered a claw.
It's weird what gets blocked out or altered over time.

You were the Night Stocker guy? So you paid it forward!

[nightrnr]

You were the Night Stocker guy? So you paid it forward!

Yeah, that was me

I've always liked the "guess this game" topics though.