Chris,
Thanks for your post. There are no known issues with LV 7.1 and 8.0 using the same code. LabVIEW has always been designed with backward compatibility to allow you to open any previous VI in the newest version of LabVIEW.
Does the VI work fine in LV 7.1? It is possible that if you have multiple applications open at the same time that there may not be enough memory available. Try looking at the Task Manager in Windows to see what the memory status is on your PC. Also, try opening LV 8.0 by itself and open another example (may DAQ, etc) to see if there are any other issues there (it may be that there is an issue with your LV installation).
Thanks,
Scott Savage
National Instruments
Applications Engineering