It's not severe, lasts like 1/10th of a second, but occasionally it's enough to throw me off and ruin a race. I've been on break from the game for a while, so I'm not sure if it's this most recent patch causing it.

I seriously doubt it's my hardware, very high end. Playing at 2560x1080, no mods/addons at all, just the HD texture pack.

This occurs offline as well. The game seems to want to peg one thread to 100%, and the other 11 are just sitting completely idle. Very frustrating.

Any tips are welcome. Hate to make a new thread but searching hasn't yielded any results that help.

The game is not multithreaded, so it's not going to use more than one core. The inner workings of the game predate the ubiquity of multicore CPU's.

Does the stutter go away without the texture pack?

If you're running Win7, open Resource Monitor (not sure if it's called the same thing in other versions) and keep an eye on the CPU and Disk tabs to see if there's a particular process doing heavy CPU or I/O access.

Running Win10, it has worked very well in the past. HD Texture pack seems to make no difference. Here's a view of task manager, nothing out of the ordinary.

1.2ghz? check your windows power saving/performance options.

Does this happen just online or even offline? One possibility would be that your computer clock is running slightly faster and LFS compensates for it by pausing itself for a little while when the time delta exceeds some threshold.