It would sound like you're running out of fuel, except your post claims you have plenty of charcoal still available. I'm just taking a stab since I can't actually see your pit, but I'd have to say somehow you're having a significant airflow problem, and there's also a possibility of damp charcoal.
How far open is your top vent?
How far open is the dampner that's housed in the guru fan?
What kind of charcoal are you using?
Is that charcoal fresh?
Where do you store your charcoal?
18.5" WSM, Large BGE, Backwoods Party
Blue BBQGuru DigiQ DX2
Blue Superfast Thermapen