I have a convection oven that has been working perfect for two years. Triphase electrical. Work with spongy cake. Recently, couple of my cakes after baking, have been sinking, the top sinks and the bottom is very dense. 

I am using same settings, temperature, time, baking mix, baking pans. From the same rack, one sinks the other does not. ??????

With extra thermometers measured the temperature, variation is not over 10 degrees

Could it be the variation of tri-phase power in the building or should I call technician to service the oven?

In the meantime, should I raise the temperature 25 degrees to help my baking?

Thank you, I am very anxious with this issue..