multiple interfaces problems

I have a problem setting both the IF_ETH0 and IF_PPP3 interfaces on.
If I use only the IF_PPP3 interface I can connect to internet using GPRS modem. If I configure also the IF_ETH0 interface the modem connect to internet but the connection goes down.
I’am using DinamicC 9.62 with rabbit rcm3700.
Please someone can help me?