Get a couple of oven thermometers to check if the oven is heating evenly. To me it sounds like one part of the oven is hotter than the other. Maybe you need to rotate the pan half way through the baking time.
I have a gas oven and don't have any problems. In fact, I just got an 11 x 15 cake done. I would have someone check out your oven....something just doesn't sound right.
I have baked in gas and electric ovens and have not noticed a difference in uneveness, only when baking in convection ovens.
First thing I would do is get a level and put it on your baking rack, the one that the cakes usually bake on. Just because your stove on top is level will not mean the rack is.
If it is not level then just level up the oven and try again.
If it's not that... let me do some more thinking.
Did it bake up above the pan? If so, it's best to level it before you take it out. It would still be a full 2".
Oven needs to be leveled and calibrated.
To help with the uneven baking...make sure you rotate the pan during baking so that both sides get baked evenly...just make sure you dont juggle the pan to much or you will pop the airbubbles in the batter and there will be a sink hole....and putting the smaller cake on the thinner side isnt really a good idea ..seeing as it probably will just sink and make it look more noticable than it would have originally...
Hmmm, I'm going to put a level to the rack. It's a wall oven (no legs) and I'm also going to rotate and see if that fixes my problem.
It did not bake above the pan. It started to on one side, kind of looked like a wave. Odd
When I did my final course I cake with Wilton I almost cried. My parents gas oven was really messed up. It wasn't level and one of the heating elements wasn't working properly so you can imagine what the cake looked like . My dad had to take out the bottom drawer to level it. The warranty fixed the heating prob. Don't worry you'll get it, its NOT you, its the oven.
one last thing not mentioned, make sure your oven is clean. you could have grease build-up more on one side of the oven than the other causing that side to heat up more, cooking that side of the oven faster. i had this problem in my last oven because i didn't clean it often enough.