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As already noted by Paolo, ordinary pings can be inaccurate. Their principal purpose is to determine if there's reachability. As he also described, performance testing software is what you really need for accurate latency measurement. BTW, many of the later Cisco devices support special SLA testing which will also provide much more accurate information about performance.
That noted, highly variable ping times, as you note, may be indicative of transient congestion. What you might also do, for every network device port along the path, check for interface drops (which often go hand-in-hand with congestion).
