Oven not heating properly

[what41109]

I have a Whirlpool Gold accubake system oven/microwave combo. I just moved into a house with the system. I thought it was my cooking that was so bad. It turns out when i preheat my oven at 400 degrees my temp gauge shows the oven at 200 degrees. I turn up the temp to 450 degrees and about 15 minutes later it shows its only at 350 degrees. Not sure what would cause this. Hoping you could help shed some light on my problems. Thanks