iperf - first packet loss and acceptable jitter

I'm using iperf to test the bandwidth on my network. I have a few questions: 1. I'm running this command on the client: "iperft -c -u -b 400m" and it successfully runs. I'm running the command every few seconds. I've found that if I run them too closely to each other, the first packet loses a good chunk of data (20% - 30%). However, if I wait 2-3 seconds before consecutive commands, the first packet usually has no loss. Is this an issue with iperf and my PC, as opposed to a problem with iperf and my network? 2. I'm getting jitter in the range of 0.079 ms and 0.536 ms. Is this normal and/or acceptable? What is an appropriate amount of jitter for a network to support VOIP, in your opinion? Thanks, in advance, for your help! - Charles