I just bought one last week and I have exactly the same problem.
I got some GE silicon seal and went around the firebox and sealed all the joints. Lots of leakage available into the firebox from the seams.
Replaced the flimsy door on the firebox with 3/32 steel - and then used silicon seal and wax paper and made a "gasket" for the door. If you can't replace the door, you can still use the silicon seal/wax paper system to make a good enough gasket to seal the existing door.
Also used silicon seal and wax paper to make a "gasket" for the door that lifts up on the firebox.
NOW - still am having same problem - so tonight am going to go around the cooking chamber with 1"x1"x1/8 angle iron and start sealing up the horizontal joint on the cooking chamber.
I put some oak pellets in it the other night and watched where the smoke came out to identify all the places that the firebox/smoking chamber leaks.
I am hopeful that once I get this thing sealed up that it can be made to get up to temp and hold it.
My thought it that there is just too much heat escaping from all these leaks and that it's not staying in the cooking chamber long enough to build temps to a cooking level. I also fooled around with the chimney - full open, full closed - didn't seem to make much difference.
Even using the BBQ Guru - I couldn't get it up to temps.
If you are relying on just the suction from the stack, then sealing that cooking chamber is going to be critical in order to make the air come thru the firebox first. Also, try taking some heavy duty aluminum foil and make a stack extension - just for a test. The higher the stack, the better it will pull.
I also am going to put some sand or bricks in the bottom of the cooking chamber to add to the mass of the cooker - and hopefully retain some of the heat.
Can't wait to get off work and into the shop and start welding on this sucker. I think it'll be a good cooker once we get the airflow problems worked out.
I also added a shield inside the smoking chamber to get the heat from the firebox moved more toward the middle of the cooking chamber, - that did even out the temps some.
Send me an email at firstname.lastname@example.org
and let's compare notes.
Hope that job comes thru - keep up the faith man.