[Overdrive]

If you're treating the throttle like an on/off switch in that situation, it's probably just a result of your inputs being too extreme for the situation and/or, as BYprovider mentioned, your new tires are not yet scrubbed in. Until I've put at least 500 miles on new tires I drive very carefully on them.

Anyway, getting back to your driving and how that may be the factor: if you were already engaged in the turn , the car's pretty much gripping and content, and then put your foot down, the weight transfer is going to cause the front to get light, and the nose is going to understeer, which it sounds like is what happened to you. Then if you just lift your foot off quickly instead of gradually (a reaction that makes sense to our minds but not the car), now you've suddenly thrown the weight to the front, the front tires will snag again, and now the rear is risking getting light and stepping out. Meanwhile you're probably still overcorrected for the understeer you had with more left (in this case) input than is needed, trying to force the car to turn left, so if the rear does start to slide when the car suddenly does start coming back to the left, you're already making your need for recovery worse.

To me it sounds like this is what happened in your situation, but the fact that you only seem to be experiencing it in lefts may mean that the alignment isn't quite right. Are you sure you're able to perform the same maneuvers to the same extremes going right, but have different behavior?

That last part is the only thing that throws a wrench in my thoughts about it being a matter of gradual throttle input instead of snap inputs. Has the car ever been in any collisions while you've owned it? Was it owned by someone else before you where that could have happened?