How to cook brownies in convection oven?

[Bubblez]

Ok, about a 1/2 hour ago i tried making some borwnies for my school backsale. i followed the directions on the box exaclty, like i always have every time i've made brownies. We've recently gotten a new stove for the kitchen and its a convection oven, unlike the one we had before. It was the first time i've ever used it to bake something. Well anyway i put the brownies in and set my timer for 27 minutes as i always have on 350 degrees. a few minutes ago ( at 24 minutes) i went down to check on them and they looked a little strange.it was very dark, the edges and the batter in the middle of the pan looked almost burnt and the rest of it was bubbling. concerned that they were burning, i started to take them out of the oven but the batter started moving around as if it weren't done so i put it back and walked away for a few minutes. When the timer went off and i came back to check on them, the batter was completley messed up. parts of it were burnt it was still bubbling, and water or oil were all over the surface. i took it out, put it on a rack and put a tooth pick in it. it was still liquid in some parts and soild not in others (and not in the usual way as if it were uncooked), so i threw out the batter, and now i'm not sure what I did wrong or the what OVEN did wrong. Does anyone know what happened? did i do sometime wrong? is there a certain time/temp. that brownies, cakes, and other baked goods should be cooked at? please help, i'm a big baker in my family and i love doing it. thanks.