Only accept if I truly helped, I realize 30 years in repair, your problem is critical, no reason to leave bad feedback, it is bad for both.
Well you were certainly resourceful to warm up the oven with the broiler but that has to be a real job to try and regulate manually. So we have a good broiler, timer, power is probably not an issue, with no model or brand, I will start with the basics. The thermostat or switch is bad, there are some over temp thermal devices that act like cutoff switches if the oven would get too hot, those fail often mainly the older type which are bi metal switches, the newer thermistor type are more reliable, then you have the possible various problems with the element or elements themselves, one failure could be easily resolved, over time the element gets jogged from loading an unloading the oven, plus corrosion from moisture, and it could be as simple as a high resistance connection to the element[s] lower for oven. To check remove, many are just a couple screws and the element pulls out, clean the connection and reinstall. Next the element of elements could be open, or cracked, this messes the whole resistance the controls expect.
Firs thing I would do until I know what brand and type and age, and model, is to take the elements out, clean connections, visual inspection and reinstall. This can be done with oven unplugged or breaker off, or both. If oven is really new, I would unplug or trip breaker, and let the electronics try and reset, for maybe 10 minutes no power, power back up, only on really new units, but if you can try looking at the elements, try to remove clean and reseat, this is the least expensive and most possible user serviceable option, then we can talk more and go in depth, but you might be surprised just a high resistance connection can create what you are describing, look into that and get back with model, age and brand,