Dealing with an oven that can't keep a steady temperature for baking

I've been baking for years, but my recent cakes come out lopsided or burnt on one side. Using an oven thermometer, I found the temperature swings by 25 degrees. It's frustrating because I follow recipes carefully. My recent vanilla sponge was a disaster, with one side raw and the other overdone. Rotating the pan didn't help much. I've tried preheating longer, but that doesn't seem to make a difference. Has anyone fixed a similar issue? I'd love advice on dealing with an unreliable oven without buying a new one.