Slight flatspot around 3k rpm?
 

On the way back from the peak tour today i noticed a slight flatspot in the acceleration of my MY02 WRX. Nothing fiddled with the engine other than better air filter and ninja backbox.

As i was accelerating it got to 3,000rpm and power dropped with a slight shudder. As the revs dropped on gear change down it did the same. Then after a few miles it disapeared again. Mystery?

The only think i could think of may be some power boost i put in the fuel?

Any ideas to what may be the cause? Not that it matters now as its gone away, but still always nice to know.