Hi,
You are not the only person with this issue. I too am running Windows 8.1 x64 and have experienced the problem with AAT and OCR exams.
My issue has been where a user is taking an exam, they click "Next" to move to the next question following selecting an answer from the multiple choice and it will then bring up a grey box in the middle of the screen with "Loading . . ." which will hang there for a few seconds, then it will change to "Attempting reconnect . . .". That message will then disappear, but the software itself is completely frozen. It is possible to CTRL + ALT + DEL and log the user out which will kill the software and after logging back in and waiting for a minute, the software can be started up again, the user enters their key code again and it will return to the question they had taken with the answer still selected so that they can then click "Next" and continue to complete the exam. It is random though, making troubleshooting very very difficult.
I have tried online, offline, in safe mode with networking, put antivirus exclusions in place, uninstalled the antivirus . . . the list goes on and I've not had any luck.
Is this the same issue you are having?