Things that contribute to coke trees or "snakes":
1. excessively wet coal
2. excessively green coal (how long has it aged since it's been mined?)
3. Too many fines
4. draft too low
5. oversizing the stoker for the load - short runtimes
5. excessively high coke button coal (7+)
6. Too little loose ash left in firebox
7. Too little heat reflection in firebox
8. Short cycle times/ wrong anticipator setting on thermostat
You probably have a number of factors working against you:
Make sure the coal is dry (oiled actually helps prevent coke trees), free of fines, and your draft is set at -.04 on the baro. Underfeed stokers burning a higher coke button coal should be using the largest sizes the auger will run without excessive crushing and be uniform at that size - having a large amount of 3/8" coal mixed with your 1-1/4" stoker coal will make the coke too dense. A high coke button coal will tend to produce more coke during mild weather but if you solve the other problems it will still burn up. Make sure that you are leaving lots of loose ash in the firebox. the overfire air should be just cracked; too much and you cool the firebox temps and prevent the coke from maintaining combustion. make sure all firebrick is in place (add some more where you can, more reflection will improve efficiency even though you lose some heat transfer).
Try changing the anticipator setting on your thermostat to have longer cycles - find a balance between comfort and allowing the furnace to come up to temp. Too much underfire air with a high coke button coal prevents will tend to create more coke. The air adjustments on these stokers with certain coals are not easy; they are not intuitive and require not only regular observation to "get right" but also long periods of time and consistant conditions for each setting to be properly observed. Once a setting is found to be good for a particular coal and appliance it will typically not need to be adjusted whether the weather be warm or cold. Try decreasing the feed further than stock settings will allow and then further decrease the air; with a higher coke-button coal slower feed rates than will-burt initially allowed help the stokers perform better.
How big is the place you're heating? BTU load? That's a big stoker and with short runtimes you can end up with heat exchanger sooting, coke trees, big thermostat overshoots, and poor performance. I heat a pretty decent size uninsulated home in buffalo with a model 77 used as an add-on furnace quite successfully, that furnace you have is going to need a big load to keep it happy.