Why My Cake Sinks After Baking

Nothing baffled me more than pulling a good looking cake out of the oven and it deflates before my eyes.

My oven uses gas and I've never had such bad luck baking. Even when I preheat the oven and get the heat conversions right, why oh why does my banana cake look like this:

It was supposed to be done after 45 minutes of baking. I stuck a chopstick in the centre and there was gooeyness. It went back in and I gave it a temporary blast of extra heat to bring it back to cooking temperature.

It was still not done after 20 mins and smelt overcooked. It was a lost cause.

When I put my ear close to it, I heard air bubbles collapsing. The cake sank and resembled a brick.

After some research, I did a test. I whipped up a lemon cake (because I ran out of bananas) and baked at a higher heat by two gasmarks. The banana cake cooked at gasmark 5 (roughly "180 degrees"). The lemon cake went in at gasmark 7. Here it is when I took it out:

A bit overcooked on one side but I did'nt want to risk the cake collapsing by turning it during the cooking process. The insides were firm and when I listened to it, there was silence. (Am I the only person who does this?)  Here's what the cake looks like inside:

So the problem was my oven's heat settings. With a little trial and error, I'll bake a cake to be proud of!