[RESOLVED] Inconsistent Timer with GUI Library

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Option Explicit
''''''''''''''''''''''''''''''' In Userform ''''''''''''''''''''''''''''''''''''''''''''''''''''
Private iCount As Integer

Private Sub NextButton_Click()
    If enable_SM Then
        ' If State Transitions is True, proceed
        iCount = iCount + 1
        Call ProcStep(iCount)
    Else
        ' If State Transitions is False, go to Timer State. 
        ' Clicking NextButton will start and stop the timer.
        If timerStart = False Then
            Me.timerLabel = "00:00:00"
            StartTimer_State
        Else
            StopTimer_State
        End If
    End If
End Sub

'''''''''''''' Timer Processes Defined for this Procedure '''''''''''''''''
Private Sub StartTimer_State()
    Application.OnTime NextTrigger, "TimerModule.OnTimer", Schedule:=True
    timerStart = True                  ' timerStart is a Boolean that indicates if timer has started
    iStart = Now
End Sub

Private Function NextTrigger()
    NextTrigger = Now + TimeValue("00:00:01")
End Function

Public Sub OnTimer()
    Application.OnTime NextTrigger, "TimerModule.OnTimer", Schedule:=True
    ProcPerSecond
End Sub

Private Sub StopTimer_State()
    Application.OnTime NextTrigger, "TimerModule.OnTimer", Schedule:=False
    timerStart = False
    enable_SM = True
End Sub

Private Sub ProcPerSecond()
    Me.timerLabel = Format(Now - iStart, "hh:mm:ss")
End Sub

''''''''''''''''''''''''''' State Library ''''''''''''''''''''''''''''''
Private Sub ProcStep(iProcedure As Integer)
' GUI Library
    Select Case iProcedure
    Case 0
        Call State0
    Case 1
        enable_sm = False         ' Begin Timer State
    Case Else
        Unload Me
    End Select
End Sub

Private Sub State0()
    ' arbitrary code here
End Sub

'''''''''''''''' In TimerModule: ''''''''''''''''''''
Private Sub OnTimer()
    UserForm1.OnTimer
End Sub

Hi guys,
I'm fairly new to Visual Basic (and to the VB Forums - even posted this in the wrong section!), but I'd really appreciate it if anyone can help me and/or point me in the right direction. This is run in MS Excel 2007.
I've omitted irrelevant parts of the code, but I basically have a GUI Library that rotates through 5 "states". One of the states will have a timer that counts up in the "hh:mm:ss" format, updated each second.

The problem is, stopping the timer have been inconsistent. It works about 60% of the time when I'm testing it, but the other 40% I get this error:

Run-time Error '1004':
Method 'OnTime' of object '_Application' failed

And I have no idea why. Shouldn't:

Application.OnTime NextTrigger, "TimerModule.OnTimer", Schedule:=False

Adequately stop any timer procedures being executed? Why is it a hit-or-miss?