stumbling engine

i have a 1997 s10 blazer 4wd. my engine has been stumbling alot around 2500 rpm. also it is alot worse when it rains out until the truck warms up but even then if you realy listen you can still hear it. its almost like a stumbling poping sound. its been happening for a couple of years but is getting worse now. i have changed the coil cap and rotor spark plugs wires. any ideas o2 sensor, fuel injectors? also when i remote start my truck and i go outside the water that is coming out of the exhaust pipe has lots of carbon it it. no service light yet.

There might be a pending trouble code, one that hasn't met all the conditions of a hard set code yet.
You might try to grab a free scan at a chain type auto parts store.