I think your problem is you have your code in the Change event and that code is changing cells within the columns you are monitoring... than change by your code is kicking off the Change event again which makes another change to the same column thus kicking off the Change event again... this process goes on until Excel locks up or crashes. If I am right, you can solve your problem by putting this line of code as the first line of code in the Change event...

Application.EnableEvents = False

and then putting this line of code as the last line of code in the Change event...

Application.EnableEvents = True

I don't see this occurring in the code you posted, but for future information, if your code can raise an error so that you need an On Error statement of one kind or another, then make sure you execute the Application.EnableEvents=True statement before you let your error trap end the subroutine.