2000 Pontiac Grand Am Rough Idle and hard to start!

The grand am runs good I bought it off somebody and they told me it runs a little rough and sometimes stalls but they had a mechanic look at it and he couldnt find the problem because it has never gave any codes. I have to press the gas a little bit to get it to start and when idle, the rpms will drop and it will put a little bit. It has a new fuel pump fuel filter all new spark plugs that were put on by the shop and I also went and bought a throttle position sensor and put it on. No change and today the engine light finally came on and it said p0300 random cylinder misfire detected! It is the 2.4 twin cam engine and its a 2000 grand am! PLEASE HELP!