sporadic error security token could not be authenticated or authorized for webservvice call

We are getting sporadic errors  saying below stack trace from app logs, the same web service are used by many vendors and  It gives proper responses for most of the times, but when some client  call the WS repeatedly It gives below error after some time and after within seconds the next call get successfull results

Pleas let me know what would be the exact cause

MESSAGE1: The security token could not be authenticated or authorized
STACKTRACE1:    at Microsoft.Web.Services3.Security.Tokens.SecurityTokenManager.LoadXmlSecurityToken(XmlElement element)
   at Microsoft.Web.Services3.Security.Tokens.SecurityTokenManager.GetTokenFromXml(XmlElement element)
   at Microsoft.Web.Services3.Security.Security.LoadToken(XmlElement element, SecurityConfiguration configuration, Int32& tokenCount)
   at Microsoft.Web.Services3.Security.Security.LoadXml(XmlElement element)
   at Microsoft.Web.Services3.Security.Security.CreateFrom(SoapEnvelope envelope, String localActor, String serviceActor)
   at Microsoft.Web.Services3.Security.ReceiveSecurityFilter.ProcessMessage(SoapEnvelope envelope)
   at Microsoft.Web.Services3.Pipeline.ProcessInputMessage(SoapEnvelope envelope)
   at Microsoft.Web.Services3.WseProtocol.FilterRequest(SoapEnvelope requestEnvelope)
   at Microsoft.Web.Services3.WseProtocol.RouteRequest(SoapServerMessage message)
   at System.Web.Services.Protocols.SoapServerProtocol.Initialize()
   at System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean& abortProcessing)