Half Life 2 Episode 2 Stuttering

I'm playing through HL2:EP2 again, and I'm getting a strange stuttering problem. The game itself runs fine, the stuttering seems more related to the Physics aspect of the game.
For example if I pick a box up, the game will stutter for a second or 2, and the audio will loop. Or if I break/smash something, I get the same problem.

I'm playing at 19x12 with everything maxxed. When I exit the game, and check task manager, one core has been used about 100% whilst the other core has hardly been used at all.

There is an option in game under Video (I think) that says something like 'enable multiple cores' but I don't know if this means GPU or CPU. Regardless, the select-list is broken, and I don't seem to be able to select any options.

Is there a way to NOT have steam running in the background? I should have enough power to run this game maxxed out, but maybe closing down steam first would allow access to more resources. Every time I play the game I log into steam and then this runs in the background...

ANyone any ideas how to fully utilize 2 cores, and maybe reduce this stuttering?
Thanks,
J