Did you change some hardware after upgrading to Pro? Retail keys get tied to a hardware hash so if you make significant changes, you may get deactivated. But it shoudnt happen off and on.
Random deactivations can happen because of a permissions problem. Follow the steps here. Genuine Windows 7 Became Pirated? Make It Original and Activated Again - Tweaking with Vishal
I can take a look at your mgadiag log for clues. For that, download mgadiag, run it, click on the Copy tab, paste into notepad, save as .txt file and upload the file here. go.microsoft.com/fwlink/?linkid=52012