Now....the mobo throttling the CPU...that doesn't sound quite right...
Judging from my understanding of a motherboard, it is basically a plate that you stuff your components on, some have SLI/CrossFire capabilities and blah blah blah but more to the point, this doesn't sound right at all, so what I would do is the following;
1. Clean your system out - this can effect performance and cooling, you'd be surprised by how much
2. Check for viruses - always good to check for nasties that could make things run a bit slow
3. check your temps - could even be thermal throttling at this stage
4. list all of your PC components - gives us all a better understanding of whats going on, for all we know (I doubt this a lot) you could be running an 980ti on a 500W PSU (Not the best of ideas)
5. Monitor your hardware load on Task Manager - any irregularities can cause massive performance drops in games (Corsair Link sucked up about 40% of my systems RAM at one point...ouch)
Other than that, the 980ti is a VERY powerful card, one under the Titan (Correct me if i'm wrong (shouldn't be wrong though!
)) so it shouldn't struggle with Arma at all....or anything for that matter