Rough Idle and acceleration?

So i asked a question a while back regarding my 1998 Mitsubishi Diamante Rough idle and such. All it had problem with was a rough idle and acceleration. Took it to a shop and they aid all is fine after replacing spark plugs, wires, cap and rotor. They also changed the valve over gaskets. As i drove out of the parking lot noticed the idle was still rough. I i had to press on the acceleration very very slowly or else the rpms would drop and the engine would backfire. The shop was closing and said to come back later. A check engine light came on diving home and had a friend use his tester, and it gave the code p0300. Also said cylinder misfire detected/Eval misfire supported/ eval fuel system supported/ eval component supported. No clue what those mean and could really use some help. Dont wanna pay extra money i shouldnt have to.