Sounds like the same problems I once had. TPP not a bad idea. Anyways here's what I did to stabilize the game
Go to Options and change the following. Resolution 1024x768, FPS cap: 120, Camera speed: 1.7
All of this
should get rid of your lag. IF the game still lags then you need to upgrade your graphics card
TBH I'm almost at the point of upgrading mine myself as the game crashed on me during both scenes with the female alien as soon as she started to remove her armor. It's like the kiss of death. What a tease