Look up "video artifacts" using Yahoo or Google. Those are overheat-related behaviors. You may need to locate and install a thermal recording utility. When you do get back to where the game PC is, you will want to edit your original post. There was a Radeon HD 3470, an HD 4650, HD 4670, HD 5570, HD 5670, and an HD 5750, but I've never heard of any "3750" before.
P. S. Minor oops here. If you used the word "Lag" to mean animation slowdown, only the HD 34xx level of those named might be likely to do so at better resolutions, but should still at least operate if screen resolution adjustments of appropriate nature are applied.
The game had it so the video driver would stop working so I manually sped up the fan and it fixed that.
I think I have a 5750 on my desktop, I forget.
But the lag is not any animation, everything is going fine but it acts like it dosn't recognize the v button for vats or my mouse clicks, or tab for pip-boy. The movements and everything working fine though, it is weird.