Why a communication of Ethernet or I2C makes a trigger pulse jitter severer?

I have made a trigger generator with an Uno R3. It gets an trigger from external and generates a serial of triggers by an interrupt subroutine. When it communicates with PC by Ethernet, the jitter time of trigger pulse changes from 0.2us to 0.5us. If I do the communication with another Uno R3 through I2C, the phenomenon is the same. Does the timer that Ethernet or I2c is using lead to this? Or the communication will produce an interrupt time lag? But I did nothing in the 'loop program'. Do you know why the communication will make the jitter time change? Is there any way to solve this problem?

Thank you!

It depends so much on what you have done, that you need to supply much more information.

aarg: It depends so much on what you have done, that you need to supply much more information.

I have found the real reason,and there is another problem. The truth is like this. http://forum.arduino.cc/index.php?topic=395533.0

Thank you!