Browser.AttachTo<T>() causes intermittent exception "Timeout while Internet Explorer busy"

A WatiN step that runs as part of a continuous integration build process is failing while attempting to attach to a browser instance intermittently. Currently about 1 in 5 times.

Browser.AttachTo<IE>(Find.ByTitle("Title of Popup"));

With the following error:

WatiN.Core.Exceptions.TimeoutException: Timeout while Internet Explorer busy
   at WatiN.Core.UtilityClasses.TryFuncUntilTimeOut.ThrowTimeOutException(Exception lastException, String message)
   at WatiN.Core.UtilityClasses.TryFuncUntilTimeOut.HandleTimeOut()
   at WatiN.Core.UtilityClasses.TryFuncUntilTimeOut.Try[T](DoFunc`1 func)
   at WatiN.Core.WaitForCompleteBase.WaitUntil(DoFunc`1 waitWhile, BuildTimeOutExceptionMessage exceptionMessage)
   at WatiN.Core.Native.InternetExplorer.IEWaitForComplete.WaitUntilNotNull(DoFunc`1 func, BuildTimeOutExceptionMessage exceptionMessage)
   at WatiN.Core.Native.InternetExplorer.IEWaitForComplete.WaitWhileIEBusy(IWebBrowser2 ie)
   at WatiN.Core.Native.InternetExplorer.IEWaitForComplete.WaitForCompleteOrTimeout()
   at WatiN.Core.WaitForCompleteBase.DoWait()
   at WatiN.Core.Native.InternetExplorer.AttachToIeHelper.FinishInitializationAndWaitForComplete(IE ie, SimpleTimer timer, Boolean waitForComplete)
   at WatiN.Core.Native.InternetExplorer.AttachToIeHelper.Find(Constraint findBy, Int32 timeout, Boolean waitForComplete)
   at WatiN.Core.Browser.AttachTo[T](Constraint constraint)

I've done all the obvious things which have been suggested in previous posts:

I.e. Kill all internet explorer processes before running a WatiN step:

Process.GetProcessesByName("IEXPLORE").ToList().ForEach(p => p.Kill());

Increase the timeouts before running any WatiN steps:

var timeout = 60;
Settings.AttachToBrowserTimeOut = timeout;
Settings.WaitForCompleteTimeOut = timeout;
Settings.WaitUntilExistsTimeOut = timeout;

Each WatiN step is pass