When we redid the kitchen in '07, we opted for double ovens, and the top one was convection. I didn't use it every time, but when I did, I liked it.  Two or three years ago, lightning claimed that unit and we replaced it with the new edition of the same model. Both ovens are convection, and I use the option quite a bit now. The only thing that comes to mind with what you're seeing is maybe putting the food in the oven before the preheat is completed. Our ovens use the broiler element during preheat to bring the temp up quicker, and a couple times when I first used ours, I did pop the food in there before the chime went off. That's been the only food quality issues I've had. I hope yours is something similar.