smoothwall disconnects without any reason

I'm German, my english may be wrong...hope you understand me anyway.

Hello.
I've installed Smoothwall 0.9.9 Special Edition on a Pentium-S 75 MHz, with 16 MB EDO-Ram and 213 MB Hard-disk. Additional I have got 2 Realtek
RTL 8139(A) Network-Cards.
I've configurated Smoothwall 'RED+GREEN', RED is ADSL-Modem 10Mbit, GREEN is local Network 100Mbit.
After the configuration of the GREEN Network, I configurated the RED using the Webinterface.
Actually I can connect to the internet and everything works 100% correct but without any reason Smoothwall suddenly stops working and the Webinterface doesn't respond anymore. I can still log in at the Prompt directly at the Smoothwall PC and change setup etc. so Smootwall dont crashes, it only stops working.
I hope I discribed my problem good enaugh for you, to give me a sollution,
Thanks,
Matthias