change events for combo boxes

Sorry for the long post but I need to explain my dilemma in detail in the hope that some bright spark out there will be able to tell me where I'm going wrong.

I have a workbook set up as follows:

Sheet 1 contains a combo box (call it Combo1) that gets its list from a named range on sheet 3.

The Combo1 change event runs 2 advanced filters on sheet 4 and copies the filtered results to sheet1. Then, using the value selected by the user the code sets an advanced filter (call it AF1) on sheet5.

Combo 2 which is on Sheet2 has its listfillrange set to the results of AF1. I want combo 2 to run a further advanced filter to provide values for a thrid combo. To accomplish this I attached code to the change event for combo 2. The result was that the workbook kept crashing Excel.

The problem is that the change event for combo2 is triggered by a change to any value on sheet1. I've set a count to see how many times the change event code for combo 2 is triggered and it runs 44 times for each change to combo1. I would have expected it to run only once. It also runs if I type a value in an empty cell in sheet1!!!

I'm pullling my hair out here so i hope somebody can help me before I go bald!!

Regards,

Paul