Do you have a K&N oiled air filter by chance?
I'm not going to get into the whole debate I have seen about them on many forums, but I will say that I have heard if a filter is slightly over-oiled, then when it rains the humidity in the air can cause a little of the oil to come off and get on the MAF'* thermistors, I am not sure how true that is, as to me it does not make sense because oil and water do not mix, but I suppose it is possible.
I'd remove the MAF, and clean the thermistors well with some MAF cleaner, then see if there is any change.
If you replace the plugs go iridium, there are only two brands I will use personally.
ACDELCO Part # 41101
NGK Part # 7164 TR55IX
They have some other NGK'* that sound pretty good, they may be a tad better than the TR55IX'*, they are TR4A15 Laser Iridium plugs.
Hmmm, the TR55IX'* are laser welded also, so I am not sure how much difference there are between the plugs, and since I have only heard of others with 3800 II'* use the TR55IX'*, they are the only NGK'* I can personally recommend.