I've read about the RC filter I've suggested on the AvrFreaks forum I've linked some posts above.
Initially, I didn't think it was possible that a similar solution could help me but finally I got my chips working correctly.
Your analysis about the interferences between the crystal caps and the RX0 pin could be correct but I don't have an oscillo to check the signals and at the moment I've only did some experiments on a breadboard and maybe the interferenes between adjacent pin rows could disturb the serial transission.
BTW, using a 12 MHz cer resonator I could program the chip without the RC filter. Some people talked about clock issues but he did only with frequencies around 20 MHz, I'm experiencing troubles at 16 MHz, while at lower clocks the chips seems to work correctly.