The boiler has a removable baffle (the wire handle is at the top), remove it and clean off the soot. If it's missing, that is your issue, the baffle has to be there to contain the heat from the flame.
Also, check the upper outside condenser (above the sloped tubes), is it warm, should be. The freezer cools first, then the fridge, always. You might need some auxillary fans to pull the air through the heat exchanger unit as well.
Finally, are you running the fridge level (within a half bubble on a mechanics level)? Ammonia fridges have to run in a fairly level position to work properly.
