On you initial posting you mentioned something around..."ping is working for few seconds and then stops"
and then you also mentioned that you CAN see it in your Ethernet network and your (other IP range) router... so from all of that i take it that your setup looks like:
some IB nodes --> IB to Eth Gateway --> L3 Router --> other subnet
Is this correct?
If yes, what kind of gateway are you using? (BridgeX or 4036E or is it something else).
Also, it would be good to take a look at your L3 router logs and see what the flow of things was. you might see something like: IGMP was kick-starting few MC groups, then they all dropped (along with the reason we all want to hear about ;-) )
What do you see my friend?