Any suggestions? Everytime I open the oven door, the center of my cake caves in- sometimes just a little, sometimes more than that. Also seems softer. Looks great until I slowly open the door when time is up ( even with extended time). And if it doesn't fall then, it does once I gently test it with toothpick. I have heating coils in the cakes ( 10", 9" and 1/2 sheet all have done this). I don't think I over mix. Currently, I am using the WASC recipe and it happens each time with this. The instructions say 325 degrees and bake until done. I have estimated and usually takes 35-40 min. I also tried to overcome this by baking at 350 for lesser time, still had a problem.