_IENavigate & _IEWaitLoad error

Hello,

I am trying to use AutoIt for opening webpages and taking screenshots from it. I use following code.

$url = "XXXX"
_IENavigate($oIE, $url)   Sleep(1000)   
_IELoadWait($oIE)   _ScreenCapture_Capture(@ScriptDir & "\" &  $sitename & "_CPU.png")
Sleep(500)

However, when I try to use it more then once to visit different site, I get error:

--> IE.au3 V2.4-0 Error from function _IENavigate, $_IEStatus_InvalidObjectType

--> IE.au3 V2.4-0 Error from function _IELoadWait, $_IEStatus_InvalidObjectType

Can anybody help me with this problem? Thanks

M.