I've had it serviced before and the parts took a long time to come in. I normally only use the convection bake feature. Now at 350 (conv bake) it pr-heats well them temp drops to 250. Same with bake. I now am forced to use convection only. What temp should I set it for if directions call for a particular temp…25 or 50 degrees less? Thank you!!!! PS I'm also be making Anne Seranne's rib roast on Christmas.