Major lag on single player

I am running a GTX950 and while playing, every time I get close to a group (on single player) My PC lags to 5 or 10 FPS. When group is killed it goes back up to 50+ FPS. I have played D2 LOD Many times before on this PC with only change I went from 8 t0 16G ram. Skyrim, WOW and Tomb raider all run with no lag. Any Ideas welcome.

I don’t play multiplayer.

Update. I uninstalled and reinstalled the game and it seems to be running fine now.