Hey guys,Hoping to get some help as I think I'm hitting a wall.

I have a 2008 G35x sedan with 56000 miles (90000km) US vin. Has a HR engine from what dealer has told me since I read in a post about DE and HR engine.So engine lights on and have a code p0021
variable timing bla bla. Started off by changing the oil cleared it and it came back. So looked on forums now I've changed the cam shaft sensor on drivers side (bank2) and 2 days later came back again same code. Hoping that someone can help me guide me or something instead of taking it to the dealer.

Symptoms :

1) on idle/ red light : car idles around 700-800 rpm at red lights and sometimes while sitting rpm drops to about 400ish then picks back up so rough idle

2) sometimes car has trouble starting. Takes 3 - 4 times then starts. On last start before starting the idle goes as if it's going to turn off then shoots up

3) this happens randomly at different speeds, when I'm driving the car sometimes jerks or almost like cuts off power for a second and jerks I asked my mechanic buddy says loss of power. Wife has a Pontic with tranny slip and it feels nothing like that so it's not that thank god.

im hoping someone can help me I would really appareciate. First post so hoping this is enough info. Thanks in advance