Jitter is the variation in time between data packets arriving, caused by network congestion, timing drift, or route changes. Jitter can lead to packets arriving out of order, causing choppy audio in VoIP calls. In contrast, a faulty NIC usually affects the entire computer's network connection, not just voice call quality. Signal strength primarily impacts wireless connections, and while poor signal strength can cause various issues, it does not explain the irregular voice break up if the signal is consistently strong. Additionally, outdated firmware could cause problems, but it is less likely to affect packet timing and result in jitter.