codes P0300 and P0302

service engine light came on and I went to autozone and these are the codes they came up with. My wife said at times the van will idle rough and she saw black smoke at the tail pipe wile stopped at red light. What do you guys think? It is a 97 grand voyager 3.3L

update on problem. Wife went to store and almost didn't get back home. When she pulled up I could smell the unburned gas fumes and I saw the black smoke. She said that when she left the house the service engine soon light went off and the van ran good untill she got back in it to come home and the check engine light came on and the van started running rough. I pulled the front 3 plugs to check the gap and they were all worn to about .065. tag under hood says .050. I regapped the front 3 plugs and drove it to autozone to have the codes read again. The van ren great. The code read P0202
P0300, Po302, and P0304. It ran good all the way home.

P0300-multiple cylinder misfire
P0302-misfire cylinder #2
P0304-misfire cylinder #4

Well I found the problem with my 97 GV 3.3L van. I had a couple of bad fuel injectors and I found a complete set of 6 new Chrysler fuel injectors on ebay for $90.00 + $7.50 s&h. My wifes van was running very rough and pouring out black smoke at times. The check engine light came on and then started blinking on and off all the time. I took it to a friend of mine because he had a lift to put it up on and while he was in there I had him replace the plugs, wires, PCV valve and the upper o2 senser. The van runs great now. I just thought I would let you know.