Thank you very much Bill!!
I'll include your suggestion in the program and I'll try it. Yesterday I modified the flush function and also the buffer size as in 0023 version (when I finish the whole program with gps and gprs I'll use a 644PA so I have lot of memory available). I tryed it and the flush function acts as it is expected.
Due to the strange behaviour of the arduino (gprs did not detect the at command but if I connect TX directly to FTDI it is detected in serial monitor), I decided to try it in a breadboard 644PA 8mhz and...it works very good!!!! The gprs module respond to all at commands. I can understand it. How is that possible? Could it malfunction the tx pin of my mini?
Thanks for your help, although arduino is relatively easy, sometimes it's frustrating for a newbie!!!!