03 ES V6 stumbles

My issue is now that when I'm driving around 1500 rpms after it's shifted into fourth gear the engine starts stumbling, it happens a lot up hills. I put new plugs and wires in and noticed there was oil around the old ones. I checked again after I put the plugs in a week later and no oil, guessing from it being over filled and than having it properly filled, so I continued to put the uim back on. I replaced the rotor and re used the cap since it looked fine also. I also have the battery light on and put a new alternator on since the old one was charging a little high. Any help would be greatly appreciated.