We see this type of post alot this time of year, where guys have trouble maintaining a steady temp and/or reports of temp surges for no apparent reason.
My opinion is the charcoal has absorbed moisture from the humidity in the air, in most cases.
To solve the problem take the charcoal inside the house for a couple days and leave top of the bag open so the moisture can evaporate. After the charcoal is good and dry again you'll need to keep it away from anything that condensates or vaporizes water. Keep the bags off any concrete or dirt floors, by storing them on a pallet or wood runners so the air can move freely around them. When your not taking charcoal out of the bag the top of the bag has to stay closed or it'll drink water rapidly. I always roll the top down and set a chunk of wood on top to insure it don't unroll itself, then I cover it with a flat, empty, bag for extra protection.