I'm getting this exception when trying to call external WS that
requires client side certs. It fails sporadically and not always -
Please see the below exception. This is urgent and need some immediate
assistance if possible, thanks.

The underlying connection was closed: An unexpected error occurred on
a receive. ---> System.IO.IOException: The decryption operation
failed, see inner exception. --->
System.ComponentModel.Win32Exception: The certificate chain was issued
by an authority that is not trusted
  --- End of inner exception stack trace ---
  at
System.Net.Security._SslStream.ProcessReadErrorCode(SecurityStatus
errorCode, Byte[] buffer, Int32 offset, Int32 count,
AsyncProtocolRequest asyncRequest, Byte[] extraBuffer)
  at System.Net.Security._SslStream.ProcessFrameBody(Int32 readBytes,
Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest
asyncRequest)
  at System.Net.Security._SslStream.StartFrameBody(Int32 readBytes,
Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest
asyncRequest)
  at System.Net.Security._SslStream.StartFrameHeader(Byte[] buffer,
Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
  at System.Net.Security._SslStream.StartReading(Byte[] buffer, Int32
offset, Int32 count, AsyncProtocolRequest asyncRequest)
  at System.Net.Security._SslStream.ProcessRead(Byte[] buffer, Int32
offset, Int32 count, AsyncProtocolRequest asyncRequest)
  at System.Net.TlsStream.Read(Byte[] buffer, Int32 offset, Int32
size)
  at System.Net.PooledStream.Read(Byte[] buffer, Int32 offset, Int32
size)
  at System.Net.Connection.SyncRead(HttpWebRequest request, Boolean
userRetrievedStream, Boolean probeRead)
  --- End of inner exception stack trace ---
  at System.Net.HttpWebRequest.GetRequestStream()