thread stuck in send()

I have a multi-threaded client app where each client thread does
something like this:

TestThread()
{
  SOCKET sd= socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  /*...*/
  while (running)
  {
    DWORD dwStartTime = GetTickCount();
    send(sd, pBuf, BufSize, 0);
    DWORD dwTotalTime = GetTickCount() - dwStartTime;

    TRACE(_T("Socket %d took %dms\n"), sd, dwTotalTime);
    //... calculate throughput and record results ...
  }
  /*...*/
}

The trouble is that when I watch the throughput for each test, I'll see
that socket 1 will be running for a long time, and then socket 2 will
take over for a while, and then back to socket 1. The beginning of each
time a new socket takes over will be marked with a really long time
(like 13+ seconds for a chunk of data that should take around 100ms).

For example:

Socket 1 took 85 ms
Socket 1 took 56 ms
Socket 1 took 110 ms
Socket 1 took 96 ms
Socket 1 took 202 ms
Socket 1 took 106 ms
....
Socket 5 took 11098 ms
Socket 5 took 97 ms
Socket 5 took 83 ms
Socket 5 took 81 ms
Socket 5 took 102 ms
....
Socket 1 took 14021 ms
Socket 1 took 85 ms
....

So, why aren't the threads alternating nicely with each other? I would
expect to see:

Socket 1 took 85 ms
Socket 5 took 102 ms
Socket 1 took 93 ms
Socket 5 took 91 ms
Socket 1 took 94 ms
....

Thanks for any insight,
 PaulH