I changed a little of your code (below), ran it against our zip code table (43,000 records) with a RowSourceType of Alias, and I can reproduce the problem. If you select one of the top records (value changes), then drag the scroll bar down and select one of the bottom records (value usually changes), then drag back up to select a top record, the value does not change. I have done this also on a table/cursor that only had 300 records as well. I am using VFP9 SP2.

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PUBLIC oform1

oform1=NEWOBJECT("form1")
oform1.Show

CLOSE DATABASES 

USE "c:\FMSRun\Prompt\Zip_Code" IN 0 ORDER Zip_Code

DEFINE CLASS form1 AS form


	Top = 0
	Left = 0
	Height = 250
	Width = 453
	DoCreate = .T.
	Caption = "Form1"
	Name = "Form1"


	ADD OBJECT combo1 AS combobox WITH ;
		BoundColumn = 2, ;
		ColumnCount = 2, ;
		RowSourceType = 5, ;
		Height = 24, ;
		Left = 96, ;
		Style = 2, ;
		Top = 60, ;
		Width = 156, ;
		BoundTo = .T., ;
		Name = "Combo1"


	ADD OBJECT text1 AS textbox WITH ;
		ControlSource = "thisform.Combo1.Value", ;
		Height = 23, ;
		InputMask = "99999999", ;
		Left = 264, ;
		Top = 60, ;
		Width = 156, ;
		Name = "Text1"


	ADD OBJECT combo2 AS combobox WITH ;
		BoundColumn = 2, ;
		ColumnCount = 2, ;
		RowSourceType = 5, ;
		Height = 24, ;
		Left = 96, ;
		Style = 2, ;
		Top = 96, ;
		Width = 156, ;
		BoundTo = .T., ;
		Name = "Combo2"


	ADD OBJECT text2 AS textbox WITH ;
		ControlSource = "thisform.Combo2.Value", ;
		Height = 23, ;
		InputMask = "99999999", ;
		Left = 264, ;
		Top = 96, ;
		Width = 156, ;
		Name = "Text2"


*		RowSourceType = 5, 

	ADD OBJECT combo3 AS combobox WITH ;
		BoundColumn = 2, ;
		ColumnCount = 2, ;
		RowSourceType = 2, ;
		Height = 24, ;
		Left = 96, ;
		Style = 2, ;
		Top = 132, ;
		Width = 156, ;
		BoundTo = .T., ;
		Name = "Combo3"


	ADD OBJECT text3 AS textbox WITH ;
		ControlSource = "thisform.Combo3.Value", ;
		Height = 23, ;
		InputMask = "99999999", ;
		Left = 264, ;
		Top = 132, ;
		Width = 156, ;
		Name = "Text3"


	PROCEDURE combo1.Init
		ADDPROPERTY(this,"aItems[256,2]")
		FOR j=1 TO ALEN(this.aItems,1)
			This.aItems[m.j,1]="Items"+STR(m.j)
			This.aItems[m.j,2]=m.j+20
		NEXT
		this.RowSource = "m.this.aItems"
		this.Value = 34
	ENDPROC


	PROCEDURE combo1.Valid
		thisform.Text1.Refresh
	ENDPROC


	PROCEDURE combo2.Valid
		thisform.Text2.Refresh
	ENDPROC


	PROCEDURE combo2.Init
		ADDPROPERTY(this,"aItems[256,2]")
		FOR j=1 TO ALEN(this.aItems,1)
			This.aItems[m.j,1]="Items"+STR(m.j)
			This.aItems[m.j,2]=-m.j
		NEXT
		this.RowSource = "m.this.aItems"
		this.Value = -34 && with negatives you can select but set Value doesn't find Value into RowSource
	ENDPROC


	PROCEDURE combo3.Valid
		thisform.Text3.Refresh
	ENDPROC


	PROCEDURE combo3.Init
		ADDPROPERTY(this,"aItems[100000,2]")
		FOR j=1 TO ALEN(this.aItems,1)
			This.aItems[m.j,1]="Items"+STR(m.j)
			This.aItems[m.j,2]=m.j
		NEXT
*		this.RowSource = "m.this.aItems"
		this.RowSource = "Zip_Code.Zip_Code,City"
		this.Value = 234
	ENDPROC


ENDDEFINE