5.4SOHC

I have a problem with my stock 98 5.4 sohc. I was told it was a tune-up issue. I came across a thread that led me to this site. I am fairly uneducated regarding these motors. I changed all the plugs and a couple coils, it fixed the problem for about three weeks. My engine light comes on and off. The computer scan told me it is a O2 sensor was out, but was told that wouldn't cause it to shutter. When I pulled the plugs after one month they were a little black. It usually shutters around 45 mph. It does better when i keep the rpms above 2000(to do this i usually turn the o/d off. I have cosidered it being the air filter, fuel filter, O2 sensor, fuel pump or fuel injectors. The problem began when I was at the car wash and sprayed under the hood. THe engine was cold, so no damage there. It just cuts out at certain rpms when accelerating. I was also told that it could have got moisture under the boot.

Thanks for the help,
Trey