I have the same set up as you minus the stacker. My typical start for a long cook is a full ring of briquettes with wood burried in a few levels and then topped off with 20 lit briquettes. I set up the rest of the cooker at this point and start up the digiq and let it bring the pit up to temp. This set up typically takes 15-30 minutes to come up to temp which by then the white smoke starts to slow down and the smoker is ready for the meat to go on. I have played with the top vent at times and typically leave it fully open unless it is windy and then I will close it half way. I would look at your probe placement and if there are other things that are obstructing the air flow through your smoker. If the probe is too close to the meat it will bias your reading and if you are foiling your cooking grates it can also bias your readings.
One other area I thought of might be the fit of the guru adapter to your bottom vent. Make sure on the inside of the bowl, the top protion of the adapter seals up so that all the air provided by the guru gets diverted down.