Exactly this. Maybe look into getting more airflow over the gpu with your case fans if possible. Once the GPU throttles you will get big drops in frames. some years back I had a 7870 from Radeon that would always get to around 100 degrees in some games which usually resulted in my entire computer just shutting off. I got it a bit cooler and it went to massive fps drops instead, ended up having to just replace it. but ever since I got a case with proper cooling I haven't had any of those issues.