Hi,
I have the same thing going on with My ER34 same engine as yours RB25DET...well mine does the very exact same thing but only in the morning...when its really cold outside...5mins after i first get of the road it will backfire once it drops below 4000 rpm range then hitting third and forth little hesitation during acceleration...hmmm I havent figured the thing out but...my advice to you is let somebody else with more knowledge handle to job. it might just be the fuel lowere octane, build up in the piping hot spots like they said ..who knows...