Understanding strange voltage spiking on output of voltage regulator

Hi again,

After my last post regarding a AC-DC Power Supply design, I've put something together in Multisim and now I'm trying to make sense of some of the simulation results I'm getting.

This album shows the schematic I'm using in the first image - a typical AC to DC conversion on the top half, and an idealized buck converter on the bottom half.

The second image shows the results of a simulation, where the red line is the voltage measured just downstream of the 7812, blue is output current, and orange is output voltage. The 7812 has peaks of up to 30V(!) and also exhibits some pretty unsteady behavior.

A couple questions, then:

1. What's causing this? Is it just a result of the simulation, or true-to-form what a physical one would do? If I disconnect the buck converter and keep the same probe on the output, it's a constant 12V.
2. Can it be fixed? I guess depending on the answer from 1, what can/should be done to address this?

On a somewhat related tangent, is there a better way to model a buck converter? This one was pulled from the NI forums.

Thanks!