2003 diamante ls. P0300
 

I have a 03 diamante LS. It's misfiring at idle and it hesitates/misfires upon takeoff but if I get into it it smooths out. Sometimes it runs smooth sometimes it runs like crap. It's throwing p0300 as only code. It has 97k miles
HAS:
New timing belt
New injectors
New plugs, wires, and cap and rotor.
New MAF sensor
New TPS
New New EGR vacuum solenoid
New fuel pump
New fuel filter.

Also, when stopped but in gear it idles around 350-400, but in neutral/park ita at about 700 is this normal? And my fuel gauge is stuck on E, fuel sending unit gone bad? Any help on this would be greatly appreciated.