Stanley:
You need to make sure that R7 is initialized to a valid value BEFORE program execution flow comes to the ST: label. If you are simply jumping to ST: from the reset vector then this could explain your problem.

In a like manner, if you were changing the shown code to use R5 instead, but you did not initialize R5 to some legitimate value before the program comes to the ST: label then once again this could be your problem.

The difference you are seeing could arise from the fact that R5 and R7 have different stray values by the time the code comes to the ST: label.

Michael Karas