I have this same exact problem and this was the only place I could find it at. My game starts up at around 50 fps for a good few minutes, all is great. Then about 10 minutes later im struggling to get 20fps. Eventually after about 20 minutes i quit with 10-15 fps. At any time if i use the chat button or press escape to bring up the pause menu my fps goes back to its normal state, as long as the menu is up. This is extremely annoying and it never did this before. Haven't played the mod in a while and can't find a fix.