Magic chef gas range; oven takes about an hour or so to preheat.

It seems that the gas is not getting through properly. Stovetop works great but oven takes an hour or so to preheat. Sometimes it's an hour and a half to just get it to 350 degrees. I have used the self-cleaning function twice in the past 2 weeks thinking maybe it needs cleaning but nothing has changed. Very frustrating!. Any help would be appreciated. Troubleshooting problems in book does not address this problem.