How much thermostat "range" in oven temperature is too much?
This is fully normal. Ovens are not stable, temperature-wise, and I have frequently seen such large amplitudes in temperature. Of course, it is much nicer if your oven can hold a constant temperature, that's why some people will accept the expense of an Aga. But in principle, baking recipes can handle that. Note that from a historical point of view, people used ovens with solid fuel, which had much larger temperature amplitudes and no temperature display, and their baking goods were still tasty. Ovens are not temperature-accurate, and recipes are robust for that.  

As for the recipes which are no longer reliable, the most likely explanation is that your old oven was also inaccurate, but in a different pattern. Another possibility would be a different mix of radiant, convective and conductive heat, which means that the same recipe in the same pan can require a different time. In general, a recipe which specifies time is reliable for a specific combination of pan, oven and amount. If you bake until ready instead of waiting for a time given in the recipe, the problem disappears.