Hey there,
The main causes for this will be Bad/Outdated Drivers, Out of Date OS, Corrupt Client data/settings, Software Conflicts with other programs on the system, Or an OS/Hardware level issue such as failing hardware or Overheating.
It sounds like you have covered a good portion of what we would suggestion but without system information in the form of say a dxdiag, it can be tough to rule anything out for sure.
follow the steps there to obtain a dxdiag report and post it here (or link it from pastebin if needed) and the community can take a look to see if there is an obvious cause.