Vonage problems with Dlink di634m? Streamengine settings?

Ok heres what i have right now. I have a Dlink di634m connected to my cable modem and the Linksys rt31p vonage router connected to the Dlink. I have a 15mb down 2mb up cable connection. Now my phone quality ranges from excellent to poor. Most of the time things are fine but once in a while maybe 2-3 times a week I have problems where the person im calling cannot hear me or I start to break up. I can hear them fine but they have problems hearing me. I have the following UDP ports fowarded for Vonage 5061, 10000-20000. Are there any others that should be forwarded any TCP ports??? My Dlink router also has a QOS feature called Streamengine. It has auto uplink setting as well as manual settings. I have tried both so far both give similar results. When i set it manually do I just put in 2048 for my uplink speed since my cable connection has an advertised speed of 2mb up?? Or should I use the average speed i actually get which is more like 1.5mb up? Any other ideas of how I could fix this problem?? Im considering trying out Optimum Onnlines Boost which is a premium high speed cable connection that cost and extra 10-15 a month and gives you 30mb down and 5mb up. I dont think this will really help much since even the 1.5mb upload speed im getting right now should be more then sufficeint for VOIP since i see people with 256k upload speeeds that have no trouble. PLease help!