Available Resources Error

I'm having a few issues with a macro I have written (something I am relatively new to, I might add). Whenever I run it, I get an error message of "cannot complete this task with available resources" - I am currently using Excel 2003

What I am trying to do is cycle though a list of 401 vehicle numbers (the WBSs), using each individual WBS to filter data in another worksheet (sheet "MC01"), and then copy a range of this filtered data into a separate, vehicle specific worksheet (starting at sheet "P1" and continuing to sheet "P401").

The code would run fine when I kept the Field 9 Filter Criteria1 value a constant WBS number, but when I introduced the phrase to use the different values on the WBS sheet (Criteria1:="=" & WBS.Value), the macro would run perfectly for half the vehicles before giving the error message and bugging out.

I thought this could be solved by dumping the memory on each loop by using Set WBS = Nothing but this doesn't make any difference.

Code is as follows:

Sub Update_MC01()

Dim I As Integer
Application.ScreenUpdating = False
Sheets("WBS").Select
Range("B2").Select
Set WBS = ActiveCell
Sheets("MC01").Select
Rows("1:1").Select
Selection.AutoFilter
Sheets("P1").Select

I understand that 401+ worksheets is a lot for any workbook to handle, but the fact that if I set Criteria1 as being a static WBS vehicle number Excel has no issues with it, and everything cycles fine. It is the addition of the phrase "=" & WBS.Value that causes the hiccoughs. From my very basic knowledge of VB, all I can assume is that Excel is storing this "WBS.Value" in it's memory on each loop. Am I right in thinking that the phrase Set WBS = Nothing is the right thing to use to empty this memory?