I have a 1994 ford ranger splash 4L v6. And my truck has a miss. can u help me?

when i am goin up hills in my truck it feels sluges.. when i press the gas all the way down the slugesness goes away for about 10 sec's. then the miss comes back. i sopost to be gettin about 19 to 20 miles to the gallon, when i am only getting 15. i have changed the coil pack, sparkplugs and wires. i have basically did all the lil things to it. no one can tell me what the problem is. i have had one guy to work on it and he fixed it. well a couple days later the missing comes back. he said he thought it had something to do with the airflow sencor. well the thing is i have already changed that out with a brand new one. didnt do a thing. i have also noticed that on the left side of the motor the middle spark plug is black when the others are not. i have a lil handheld comp. and all it told me was that it was runnin rich. i dont kno what the prob could be any more. can anyone help me