You have two problems

1 suddenly going into run mode

2 running startup script in newly opened workbook

The second is not a problem, that is how it is supposed to work
If that is not what you want then your logic is wrong, it could be that you are stepping through for the exact purpose of testing that logic
If you dont want it then delete / disable the startup code in the second sheet



Problem one I have not encountered before and I will follow the thread to see if there is an explanantion. you can work round it by setting a pause point, just click in the left hand column a few lines ahead and it will highlight the line in brown and then always stop there (also useful for skipping over loops that you know are working)