From time to time I have the following problems while accessing linux box over serial cable:

1) Letters cover each other
2) One parts of output mesages are mixing onto others
3) Command line appears visible but not resposible
4) Grub or pxelinux menu interprets keystrokes incorrectly
5) Computer hangs

Is it bossible that all these problems are due to incorrect nullmodem cable configuration or wiring?

I don't think so. If u have wrong cable connection, u wouldn't be able to connect at all. I think it's software problem, not hardware. Have u tried to access it with another machine?

I have tried to access this machine both from linux minicom and windows putty.

Moreover, I have two similiar machines. Although their hardware configurations are slightly different and behavior is changing slightly when hardware reconfigure, the resembled problems accures when working via serial.

Particularily, the effect of text transposition present always.

If we will speak about text transposition: is it normal while working via serial?
Is it possible, that large number of processors of fast speed causes this behavior?

Well, how about testing the serial port between the two boxes by simply transferring data between them. not by logging in from one to another. If it worked fine, u can try sending a bit big file from one to the other. u can use cutecom to transfer the files.

If things went fine then we don't have hardware problems. !

I don't think the processing power of pcs could do a problem. unless the rate of data they send on the serial port is faster than the baud rate. I guess the buffer would overflow and u could miss some data?? That's all guessing tho..