DC Voltage Source - circuit problem
jippie's right. D2 is your problem. On the reverse cycle, you've got no regulation on the voltage/current from D2.L3's top is at "virtual ground". L3's bottom is at positive 23 volts or so. This forward biases D2. The output is the cathode side of D2 I assume. That means you've only got regulation on 1/2 of your cycle (through D1). Your regulator sees the high voltage and tries to shut it down, but the power isn't coming from the regulator side. It's coming from D2 so all it's effort to shut the voltage down is in vain.

In short, you want the cathode of D2 connected to the cathode of D1. This will force both sides of your transformer to go through the voltage regulator circuit you've created.