VBScript Code: Object required error - vbscript

Question

Guys
I just wrote a vbs code snippet to automatically post in a forum. That's just for convenience. I encountered a wired problem:
I have several accounts. When I login as some accounts and use the script to automatically post, everything is OK. However, when I login as the other accounts and run the script, I got the error:
Error : Object required "getElementById(...)"
Code : 800a01a8
Source : Microsoft VBScript runtime error
I'm sure the object exists because I get it in the source of the webpage. I feel the error occurred randomly and I cannot get the regularity.
The script is ran in a Windows 8 OS and the browser is IE9. I'm a new learner of vbs and I don't know how to debug it. So I hope someone can help me. You can give me some clue.
Here is my code snippet:
Option Explicit
Dim IEApp
Dim iURL1
Dim iURL2
Dim iURL3
Dim iURL4
Dim iURL5
Set IEApp = CreateObject("InternetExplorer.Application")
iURL1="http://bbs.dealmoon.com/thread-299027-1-1.html"
iURL2="http://bbs.dealmoon.com/thread-299195-1-1.html"
iURL3="http://bbs.dealmoon.com/thread-299018-1-1.html"
iURL4="http://bbs.dealmoon.com/thread-299015-1-1.html"
iURL5="http://bbs.dealmoon.com/thread-299014-1-1.html"
Open iURL1
Open iURL2
Open iURL3
Open iURL4
Open iURL5
WScript.Echo("Done!")
Sub Wait(IE)
Do
WScript.Sleep 500
Loop While IE.ReadyState < 4 And IE.Busy
Do
WScript.Sleep 500
Loop While IE.ReadyState < 4 And IE.Busy
End Sub
Sub Post(IE)
Dim count
For count=0 To 9
With IE.Document
.getElementById("fastpostmessage").innerHTML = "good"
.getElementById("fastpostsubmit").click
Wait IE
WScript.Sleep GetRandom(7,15)
End With
Next
End Sub
Sub Open(PageURL)
IEApp.Visible = False
IEApp.Navigate PageURL
Wait IEApp
Post IEApp
End Sub
Function GetRandom(floor,ceil)
Randomize
GetRandom=Int((ceil - floor + 1) * Rnd + floor)*1000
End Function

Answer 1

You should call your POST routine after all the HTML content is loaded, for example in DOMContentLoaded or window.onload (for IE<9).

Answer 2

You dimmed IEApp, created the object, but you are not using it when trying to get the element ID. Your With block should look like this:
With IEApp.Document
.getElementById("fastpostmessage").innerHTML = "good"
.getElementById("fastpostsubmit").click
Wait IEApp
WScript.Sleep GetRandom(7,15)
End With
There are a few other places you only have IE instead of IEApp. Clear those up and your code should run fine.