My newly acquired 2001 WRX has an odd problem and funny enough its the exact problem I had in my Nissan 180SX,that I never did manage to fix in the 4 years I had it!, though it was much worse on the Nissan.

When the car is cold or has not been driven for a few hours ,the acceleration is very responsive/ crisp and no hesitation on take of. After driving for about 5 to 10 mins the car then becomes much more noticeably unresponsive acceleration-wise and also develops a really annoying hesitation on take of?!.

I have so far.
Cleaned the AFM= with brake cleaner.
Added Fuel Injector Cleaner.
Changed Spark Plugs( NGK Platinum PFR6G)
Changed Fuel Filter.

I'm hoping this wont be like that damn Nissan and give me years of hell trying to get it to run right!?.