You also might try going to the Vonage Forum at
http://www.vonage-forum.com/forum1.html. It's not affiliated with Vonage, but there are some folks there who give excellent technical advise. There's also a VOiP speed test that can help you diagnose your problem by testing your up/download speeds, packet losses, etc., and help figure out what might be going on.
FYI, I'm using a Netgear WTG624 v.2 wireless router with a VTech IP 8100-2 wireless Vonage phone system and it's been rock solid.