huge fps drops post engine change

I play on an older laptop, but could easily handle 60fps before the engine change. Not sure why but now sometimes I will often struggle to get 30fps post engine change. l will occasionally have about random half hour random intervals where l will still be able to do 60 fine though. This seems different than normal fps hardware side drops. I run at the lowest settings, my temps are fine and none of my components seem to be overwhelmed. These abnormal drops feel as though the game thinks I have the fps limiter set to 30 (I have it set to 60). Is anyone else experiencing issues like this?