My Cakes All Sink In The Center My cakes all sink in the center and are crusty all over the outside, top and bottom. I googled possible causes for this and the most common answer was the oven's temperature. 

I tried experimenting with tweaking the temperature but still had the same outcome. When my cakes bake for the recommended time it has risen but it's still runny in the center so I let it go an extra 15 minutes. At the end of those 15 minutes the cake has sunk in the center. 

I had my temperature tested and 180°C/350°F was fine and still my cake sunk in the center. 

I'm willing to pay for having my thermostat replaced but I'm worried it might not be the thermostat that's the problem and I end up lose that money.
Lower your oven temperature to about 320 F. You need the heat to penetrate to the centre of the cake and at 350 F based on your post, the temp is too high and you're forming a caramelized crust, giving the indirect oven heat more difficulty to penetrate.  

Trust me, lower your oven temp and add some time.