Reset vs. power supply disconnect/connect

I wrote application which uses Arduino Mega2560 (genuine) and Ethernet Shield (china). This application sends periodic data to my Nimbits server on Google Apps. Application works initially fine, but after couple of hours it hangs. Strange is that pressing the RESET button doesn´t solve the problem (network communication doesn't work). I have to unplug and then plug the power supply again to reestablish functionality.

I suppose the problem must be in hardware (bad ethernet shield?). Or do you think than software bug can cause such behavior?

Thanks Petr

Could be software - the reset button probably doesn't reset the Ethernet shield.

(deleted)

If it was a problem in your sketch it would probably be fixed by the reset. My guess would be a problem with the shield. Are you checking the return value from every call to see if the shield detects a problem like a connection failure?

Thanks for your answers/suggestions. My project is written in Atmel Studio/Arduino Template Express. I'm going to insert couple of debug lines (print to serial console) and also capture IP packets to find more accurate information where the code hangs. Problem is that it takes a lot of time before the problem occurs. Anyway is it possible, that pressing of RESET button on Ethernet shield doesn't reinitialize the board properly?

Petr