problem with Gvim 7.4 on Linux Ubuntu 12.04: random number of lines

When I use Gvim 7.4 on my laptop with Linux Ubuntu 12.04,
the window is opened with a random number of lines.

My .virmc file is:

set gfn=Courier\ 10\ Pitch\ 11
set lines=30
set columns=109
set textwidth=108

and my .gvimrc file is:

set ghr=50
colorscheme darkblue

The window is sometimes opened with 30 lines (which is what I want)
and other times with 26 only.

I tried to change the value of ghr, but nothing good happened.

Can you please help me with this?

Best regards,
Marco Bernardo