Straight to the TL;DR as I'm not sure what details to include.

-Bought the house in January, at the time of the home inspection the flue (8" connection on the single flue chimney) had positive draft like a $2 whore.  It was putting out my butane lighter.
-Had a chimney sweep two weeks ago, ever since we've had negative draft.  Chimney sweep indicated the flue looked good, wasn't too dirty but he was up there anyways and might as well sweep.

Chimney/Flue: single flue, clay lined rectangular, 2' above roof line, proper cap.  Nothing else (no furnace or the like) attached to the flue.  There's one 8' connection in the basement, and another on the next floor directly above which is plugged with Roxul.

What gives?  Please let me know what additional information you need to unfuck my draft issue.  (pardon my harshness)

Happy Thanksgiving!

To keep the ball rolling..
Is it always a negative draft or only when a range hood, bath fans,  furnace return air is pulling etc..?

Not to be insulting, is there a combustion air opening in the firebox?  Is it closed or obstructed?

Did you notice any smoke spilling out of the firebox after the flue warmed up?   If you open a window does that improve the situation?

Good call

How tight is the house = no combustion air

Gents, I think we all had a simultaneous "ah-hah" moment.  My wood stove is a third source of heat.  Primary is oil-fired forced hot water.  Secondary is passive solar (quite efficient at that).

For the primary and secondary systems to be effective, the house must be REAL tight.  I have two walkout basement doors.  As soon as one of them is opened even a crack...WOOSH!  We have lift off.

I guess what I'll have to do is keep a basement window/door cracked until the flue heats enough to draw.  It's just a tough experiment game with a 2 year old, six month old and a nagging wife unit.

If you crack windows and doors you will have a steady stream of cold air crossing the room and cooling everything in the room off .

If you do not have a air source the stove will draw it from somewhere and you will again be drawing cold air into the living space.

What you need is "makeup air". In my house I have a 2" pvc pipe that enters the basement wall . crosses the basement and then goes up through the floor and ends near the woodstove in my first floor living space . Most of the combustion air that enters the stove and travels up the chimney comes thru this pipe instead of around every door and window

You need heat in the fireplace to get the draft back to proper.  I've heard of using a kerosene lamp or a bunch of candles, or even just a wad of burning newspaper.  The newspaper has a tendency to get sucked right up the chimney though.  

I have quite a good draft in my chimney/stove setup but it will reverse every so often, especially in the summer.  Even with the damper shut all the way there's a little leakage, and if it's reversed it stinks the whole house up.  The kerosene lamp fixes it.

There used to be a couple places that had short draft pipes made from cast iron that could be put in the back of the firebox of a masonry chimney.
they had an inner and outer drat controller.
Most of the time you had both closed.
For winter the outer was opened and just the inner changed.

i left at least three of them behind when I sold houses.