If you suspect that the System is corrupt due to tampering or simply because it has been around for a long time, it is a good way to rule out software problems. Of course, downloading the latest drivers is also important. It is not a matter of sending a card in good condition to RMA.
If it were a matter of the CPU, you should also see a drop in FPS. Try to measure with the min/max/avg bench of FRAPS to check the minimums and relax the details if that's the case. If it is online, make sure it is not a matter of connection.
PD: Before formatting, make an image of the System so that you do not end up in the same situation and have to plug everything in again.