A pavlova is similar to a meringue. (The additional starch and vinager change the texture a bit, but not the preparation).
The key to baking a pavlova is not baking it. You want it to dry, rather than bake.
What does this mean here?
- If it turns brown, the heat is too high. (A very slight tinge is ok...) Especially with a smallish oven, the heat elements or oven walls might be too close or hotter than your thermostat setting. Also, oven thermometers are notoriously incorrect - and 30° can be a major difference here. You could invest in a good oven thermometer, but I'd probably just use a lower setting next time and see what happens.
- If its sticky, there is too much humidity left. This might be due to you using a small oven. You want to keep the oven door slightly ajar to let the water "escape" after baking. Normally you shouldn't open the door while baking due to the risk of the pavlova collapsing, but I'd probably try leaving it slightly ajar (like for meringue) once, just out of curiosity...
Would longer beating help? I can't say as I don't know how long you are beating now. But this link describes the desired texture quite nicely.
As far as the rings are concerned: No idea, sorry. (Perhaps beating time...)