Today I have done a new test on my camera and I think to have understood why of the slowdown of the shot. The inconstant jam was caused by the overturning mirror. A side of the mirror rubbed the inner side (that one in correspondence of the release button) collecting in time dirt on the contact side. I have cleaned both the surfaces and now it seems running properly. I do not know if the mechanism has suffered a damage on overturning speediness of the mirror.
I hope my English was sufficiently clear.
Thanks again for your kind support.