Mustang Problem

I have a 2000 v6 mustang, it has recently start to act like . When you first crank it up it idles high but comes down after sits for a minute. When driving the car shakes when you give it gas. When idleing the motor shakes alot. I changed the throtlle postioning sensor and didnt change tested every cylinder and they are all firing. I put a fuel filter on it and acted a lil better and when i run a high octane gas with a bottle of lucas. But one day itl run really ruff the a couple of days later itl run pretty smooth its been back and forth. I think its the fuel pump going out but thats an expensive part and want to make sure before i buy one. Also the engine light is on and its been on for awhile. I put a new camsensor in and it aint gone off since had the code reset many times but comes bak on could a cam sensor cause this problem?