That is a very common phenomenon in offset smokers.. where the heat comes from the firebox into the smoking chamber there should be a sort of baffle to help disperse the heat and hot exhaust gases more evenly.
The best modification is to install a metal baffle - by building a deck that will fit above the heat intake area from the fire box that would fit below the grating that would disperse the heat in a more even fashion. It should extend to nearly the full length of the smoker. I have seen some which have vent like cutouts all along the length of the baffle, smaller cutouts closer to the firebox and getting larger as you go along toward the opposite end.
Alternatly, you can also fabricate a rack to hold a pan of water right where the firebox opening is, blocking the heat, and instead heating the water which creates 212 degree steam which will also help regulate the temperature in the smoke chamber.
You do not have to spend a lot of money or even weld this baffle in place.. a lot of these baffles are fabricated from simple metal flashing and bent into shape by hand and installed with just a few bolts, or possily created to just rest in place.
Hope this helps!
