Is there something wrong with my cleanup code? (OpenGL + SDL)

Question

I think I have a bug in my program. I use SDL and OpenGL to render an animation. The program also measures the average FPS. Tipically, when I run the program, it will run at around 550 FPS.

However, if I start a second instance of the program, the FPS drops for both at around half (220 FPS). The strange thing is that if I close the first instance, the second one will still run at only 220 FPS. This leads me to believe that my cleanup code is somehow flawed.

Sometimes, even if I run a single instance, it will run at only 220 FPS, probably due to a previous instance that failed to clean up properly. Is there something wrong with my approach below?

I use a screen class which has the following *tors:

namespace gfx
{
    screen::screen(const settings& vs) : dbl_buf_(false), sdl_surface_(0)
    {
        if (SDL_Init(SDL_INIT_VIDEO) < 0)
            throw util::exception(::std::string("Unable to initialize SDL video: ") + SDL_GetError());
        if (!set(vs))
        {
            SDL_Quit();
            throw util::exception("Unable to setup initial video mode.");
        }
        glewInit();
    }

    screen::~screen()
    {
        SDL_Quit();
    }

    bool screen::set(const settings& vs)
    {
        SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);

        Uint32 flags = SDL_HWSURFACE | SDL_OPENGL;
        if (vs.full_screen) flags |= SDL_FULLSCREEN;
        sdl_surface_ = SDL_SetVideoMode(vs.size_x, vs.size_y, vs.bpp, flags);
        if (!sdl_surface_) return false;

        settings_ = vs;

        int db_flag = 0;
        SDL_GL_GetAttribute(SDL_GL_DOUBLEBUFFER, &db_flag);
        dbl_buf_ = (db_flag == 1);
        return true;
    }

    // ...
}

Also:

int main()
{
    try
    {
        gfx::settings vs = {800, 600, 32, false};
        gfx::screen scr(vs);
            // main app loop, render animation using OpenGL calls
            // loop runs while running_ variable is true (see below)
    }
    // catch, etc.
    return 0;
}

If it makes any difference, I use Linux and an ATI card.

Update: Event handling code:

SDL_Event event;
while (SDL_PollEvent(&event))
{
    switch (event.type)
    {
        case SDL_KEYDOWN:
            if (event.key.keysym.sym == SDLK_ESCAPE)
                running_ = false;
            break;
        case SDL_QUIT:
            running_ = false;
            break;
        default:
            world_.process_event(event);
            break;
    }
}