why that 74LS04 is needed?
to make the signal stronger
could i do that with some transistors
perhaps but transistors would not be as good because you would have to design the circuit properly to be as good as the 74LS04.
isnt any problem with that
How do you know, have you looked at the signals on an oscilloscope?
my application became more stable when i decreased the gap of flat cable (sin,latch,clk), is there any solution/upgrade on that?
Yes put the buffers in between the gaps, that is in front of each bank. Use 74LS14 in place of 74LS04 as they are a bit better coping with noise.
I used a multimeter to see the voltage on my PCBs and i saw that it changes value all time between 3.5(!) and 5 volt, why there is so much drop?
A multimeter is not the best thing for measuring this you need an oscilloscope. It could be that you have a lot of noise on the power supply lines caused by lack of decoupling. Or it could be a poor ground connection. Try running the power and ground from each bank directly back to the arduino instead of chaining them.