If you're baking your cakes on different levels of the oven, that probably accounts for the difference.
Preheat your oven for a long time, that helps to even out the side to side variations.
The convection fan will even out the heat distribution quite a bit, but some people don't like it for pastry. In your case, worth a try. Remember that you'll have to lower the heat and/or reduce the baking time.
Another trick to even out variations is to place a dish full of water on the bottom rack. If you do use the water pan trick, make sure that your oven is completely up to temp, before putting your cake in.
Since bundt cakes aren't particularly delicate and can stand a little handling and an open door for a minute, try rotating your pans so that each cake cooks for the same amount of time in the same parts of the oven. If you are baking on different levels, this is a must.
Always check to make sure the cakes are completely cooked with a trussing needle or a bamboo skewer before removing.
Finally, when you cool your cakes, use the "vestibule" method. That is, turn off the oven, slide the cakes' rack out, slide the cakes to the front of the rack so they're above the oven's open door, and leave the cakes on the rack for at least half an hour before cooling them -- still in the bundt pans -- on a rack. Don't turn them out until the pans until are cool enough to handle comfortably by hand.