GDI Drawings freezing after a few minutes

Currently in our program we create bitmaps to use on picture boxes to draw graphs and sketches.  We create a bitmap and then use the creategraphics function to begin drawing, creating a new bitmap and disposing of the old when we need to resize.   We then set the Image property of the Picturebox to the bitmap after completion.  The problem is after a few minutes of the program being up occasionally the graphics stop drawing.  They just freeze at the last drawing and stop updating.  Sometimes the Picturebox displays the red X and occasionally we get an unhandled exception in the ondraw command of the picturebox, which we don't use, but most of the time it freezes and just stops drawing.  What could be causing this?  Is there anything we can do to prevent this?

frmMain1.TensionChart.BackgroundImage = Nothing
If Height = -1 Or Width = -1 Then
	MainBelt.CurrentCase.TenPlotDrawer.Resize(frmMain1.TensionChart.Width, frmMain1.TensionChart.Height)
Else
	MainBelt.CurrentCase.TenPlotDrawer.Resize(Width, Height)
End If
frmMain1.TensionChart.BackgroundImage = MainBelt.CurrentCase.TenPlotDrawer.nImage
MainBelt.TenPlot(MainBelt.CurrentCase.TenPlotDrawer, MainBelt.CurrentCaseIndex, Color.White, , , False)
frmMain1.TensionChart.BackgroundImageLayout = ImageLayout.Zoom
frmMain1.TensionChart.Refresh()

TenPlotDrawer is  structure with a bitmap, and a graphics class along with a brush and a pen.  Its just something to bundle them all together.  TensionChart is a Picturebox.  Resize on TenPlotDrawer just resizes the bitmap creates a new graphics

Public Sub Resize(ByVal Width As Integer, ByVal Height As Integer)
	Try
		If nImage.Width = Width And nImage.Height = Height Then Exit Sub
		If Not nImage Is Nothing Then nImage.Dispose() : nImage = Nothing
		If Not nDrawer Is Nothing Then nDrawer.Dispose() : nDrawer = Nothing : GC.Collect()
		nImage = New System.Drawing.Bitmap(Width, Height)
		nDrawer = Graphics.FromImage(nImage)
	Catch ex As Exception
		CheckValid()
	End Try
End Sub


Public Sub CheckValid()
	Try
		If nImage Is Nothing Or nDrawer Is Nothing Then
			nImage = New System.Drawing.Bitmap(300, 300)
			nDrawer = Graphics.FromImage(nImage) : GC.Collect()
		End If
		If nImage.Width = 0 Or nImage.Height = 0 Then
			nImage = New System.Drawing.Bitmap(300, 300)
			nDrawer = Graphics.FromImage(nImage) : GC.Collect()
		End If
	Catch ex As Exception
		nImage = New System.Drawing.Bitmap(300, 300)
		nDrawer = Graphics.FromImage(nImage) : GC.Collect()
	End Try
End Sub