I am starting week 3 of Atkins and all has been really well. Tonight my heart has started racing. It slows down and starts up again. Its kinda freaking me out.

Anyone else experience this?

This has happened to me in the past, pretty much always stress related. I haven't had it since I left my old job about 10 years ago.

If it keeps happening you really should see your doctor just to be sure everything is okay.

I get stuff like that on occasion. Too much caffeine will do it, (like it I have a coke or something) and so will my asthma meds, stress of course. My doc says its a common perimenopausal symptoms, as are palpitations.
But if it continues with cutting out common causes, get it checked. Anemia is another possibility and there are many medical reasons, like thyroid problems, that other than a passing thing, should be seen by your doc.

I used to get heart beat fluctuations when I drank tea. I also got them while my body was adjusting to being in ketosis when I lowered my carbs enough. It went away after a few days.

Stress, whether physical or mental, will change the heart rate, too.

Hope yours is happily steady now!