I own a 2006 WRX Limited with 4EAT and 67k miles on the odometer. I have been experiencing hesitation between 20mph and 40mph. I haven't noticed any hesitation when downshifting. It only seems to be during upshifts. I even noticed that 0-60 times are consistently 5/10ths of a second slower in this car than my 2003 WRX with 4EAT which had the same amount of miles as I have now.
I've disconnected the battery and reset the ECU, but that didn't fix the problem. The dealer suggested a tune-up and a complete fuel system clean. Is this necessary? What could the potential problem be?