very same thing happened to me: https://bitcointalk.org/index.php?topic=1063402.0 (i removed the pictures but right now my miner is running at 53/45 - nearly winter in southern hemisphere + air conditioned room)
found no solution to it
i also had ~12c difference but i switched the side covers place with each other + put it over something with no hole. if u are placing it on something with holes under it to "let it breath" is actually worse. i put it over the foam that comes with the box to protect it so it doesnt shake a little bit the table. after doing these two things i now have avg temp diff ~9c.

mine is also new model with heatsink. brand new 1300 g2 powering only one miner
i will eventually put another fan in it to help with the noise + temperature i think. might eventually repaste thermal paste too