VB Code Error When Workbook Loses Focus

Overview: I have a workbook named MainPage.xls. This mainpage has several buttons and serves as a switchboard to launch other workbooks. Attached to this Main Page are two worksheets, "Source" and "Value1". The "Source" worksheet is graphical in nature and is used to collect data via a DDE channel to some production industrial processors and then on a timer "log" the data to cells in the "Value1" worksheet. Once the VB code is launched from the "Source" worksheet (via a start button), the code continues to run until it is stopped (via a stop button). The data is then graphed and printed out, etc.

This works very nicely and provides "live" graphing of some constantly changing data.
Once the "Source" worksheet data VB code is running, I need to be able to minimize the worksheets and return to my Main Page and further be able to open any one of the other workbooks on the Main Page and work with that page(s). Thus far, I can minimize the "Source" and "Value1" worksheets and return to the Main Page and the VB code runs along nicely in the background.

However... IF I attempt to open any other workbook from the Main Page, my data collection/graphing code stops and I get a Run-time error 9, subscript out of range... and Excel completely shuts down.

I know this must have something to do with the worksheet that runs the VB code losing focus when another workbook is opened.