If you were de-accelerating in the curve you probably induced oversteer. Try taking the same corner without having to slow for one of darn slugs. If everything is fine, then that was the problem. When you brake or let off the gas ("lift") into a corner you shift weight from the back end to the front in a major "weigh" (pun intended:D) causing the tail to swing out. It takes practice, but the way out of this situation is to add judicious amount of throttle to transfer the weight back to the rear to give it bite and power around the rest of the corner. Too much throttle and you can make a spin out even worse.

My next bet would be along everyone elses thoughts regarding incorrect tire pressure as the most likely culprit. If you have new tires on one end of the car but not the other and the tread difference is significantly different OR the tires are a different brand this will cause major handling problems too.

Chuck