hi,
i've changed to external pullup; the problem seems to be solved now!
it's not an option to replace the unshielded cable. there is a lot of cable i have to replace then
thanks a lot everyone!!
@dlloyd; this is still the same project, i've rebuild everything; the first version gave a lot of trouble with the freezing I2C.
I Changed all of the boards to Arduino MEGAs; so all the in&outputs are on the board now; i don't need shift registers anymore and have more Serial ports
I got rid of the I2C and now using RS485 so i can handle all the data by myself (it doesn't freeze anymore)
All the switches are handled by single ATMega328 chips which handles the push buttons;
because when using the serial interface, it sometimes don't detect all the double or tripple clicks of a button.
changed the configuration of all the switches in the house using LM339 IC's to let them operate on a higher voltage (because nowhere in the house except for the RS485 is shielded cable)
I still have some little trouble but step by step things are getting better 
Reinald