Do you have a significant number of forms/instances in your 'ODK Briefcase' folder? If so this could be the source of the problem. I would open and run with more ram using the cmd by allocating more ram
"C:\Program Files (x86)\Java\jre1.8.0_65\bin\javaw.exe" -"Xmx1024m" -jar "ODK Briefcase v1.x.x Production.jar"
Alternatively you can remove existing forms/instances from the "ODK Brieface" folder (just place them in another folder/archive). This is the better solution I have found.