My top oven turns on and heats fine but as soon as it hits temp it begins to drop. The display reads it is at temp but the actual temp falls until it ends in the 200 range. If I set a new higher temp it will immediately display the correct temp and again heat until it reaches it goal the again begins to fall. My Fridgidare certified tech has replaced the temp sensor and display board at a cost of over $400 and it still doesnt work. Help appreciated. I cant just keep replacing part after part with no improvement. The bottom oven works fine. The unit is less than 2 years old