Dont know if you have already looked into this, but heres the issue I have:
During Starbase Fleet Defense, all is well and runs perfectly. However, when I have completed the mission and leave, I begin to experience a series of bugs. For around 5 minutes after, the audio of the mission stating "incoming enemies at charlie group" for example.
Also, if i dont log out, I continue to lag, and if i dare go into another Fleet defense mission 30 mins later, the game is unplayable and registers under 1 FPS, as the lag is that bad it is literally like a screenshot every 3 seconds with audio layered over the top (the skill bar does still work though).
The machine I am using is more than capable of playing this as its specs are as follows:
Alienware M17x R3, i7 2920xm, 2GB 7970m, 16GB Hyper X Ram, 1080p display.
Always have the Graphic properties in game just below the maximum and it is always played while plugged in.