Oven preheats but then cools off

Our Maytag oven has recently decided to stop staying heated after preheating. No matter what we set it to, it will heat up to that temperature, but then gradually fall to 100 degrees or more below that temperature again. It started out only happening periodically, but now happens just about every single time we use it. The broiler works without issue, if that makes any difference. We’ve also replaced the temperature sensor to no effect. The model number is PGR5710PDW. Help please! Thanks!!!