We have a keystoker90 with a direct vent also.
The standard Honeywell (round dial) type thermostats are NOT very accurate and it depends on where it is mounted in relation to your stove. There is quite a bit of lag and inaccuracy with them.
I changed mine to a LUX digital Programmable and it has about a +/- 2-3 degree swing, since the stove takes a while to increase Temp. it can drop a couple degrees before it can catch up (I am Looking at the CoalTrol for better control!)
The Set Screw on the feeder, turn it Clockwise all the way in until snug, do not overtighten, then back it off 2 full turns and start there, you can then back it out 1/2 turn at a time to adjust the feed accordingly. I think mine is set around 2-1/2 or so.
ALSO, your Timer box, check the PIN settings, you should have approx. 4-5 pins every 10 mins. If you have more, it will cycle the stoker that much more (10-15 secs for every pin) every 10 mins to keep it burning when the thermostat is not calling for heat.
If both of these are not adjusted accordingly, they it will feed much more then it can completely burn. We do get the clumps once in a while too.
The exhaust motor housing should have a cover plate with 4 screws, you should open it and vacuum it out once a month or so and the blades on the direct vent should be checked and cleaned too!
The Combustion blower burn grate section (where the coal actual burns) can get ash under the plate and should be cleaned also, kinda of a pain, but can affect the combustion air feed.