At some point if the DNS latency gets too high, due to proxy not being able to check the DNS server or other network issue proxy will stop intercepting traffic.
You can check the sysinfo file and correlate the parameter between http:dwell:ms:dns with svc:ds:service:intercepted_server_bytes
On the DNS parameters it will show you the timings. The bigger the number the more delay it took, then you have to correlate that to the intercepted bytes. If they drop to 0 it means the proxy stopped processing traffic and the delay was caused by a DNS latency.