I've heard that boiling water will also cure polymer clay but I can't attest to it myself!
Boiling water cannot actually get hot enough to cure polymer clay. Lots of people recommend it but it simply can't work, even if the clay seems properly baked.
One thing that will help is getting an oven thermometer so you can be sure your oven is actually heating to the temperature it claims and doesn't spike badly. Don't put the pony or clay on glass because it can spike the temperature. Tenting (covering in tin foil or putting in one of those disposable foil baking tins) might also help but I haven't tried it with plastic stuff. Plain printer paper or tissue might work for tenting since paper can be baked just fine at polymer clay curing temps.