Need a bit more info to have a go at solving this:

System specs? Resolution? Frame rate? (Min, Max, Average for both mods On & Off). Gaming on Desktop PC or Laptop? What mods are you using? New install or played fine & has just started happening? Happen all the time in all weathers or only in certain circumstances? Other games play fine? Is your Mom hot? If so, what's her number? (that's a joke , probably a bad one). Any other information you deem relevant?

Give us that info & I'll have a better idea of what might be going on.

Best Guess: You don't have enough RAM. Maybe try Shader Cache: ON in Nvidia Control Panel (not sure of AMD equivalent name) if not on already.