Hesitation Around 3500rpm

[Dice]

Hey guys, I just did an 8 hour trip home yesterday arvo, in my 95 R33 GTS-T. And I noticed it all of a sudden losing alot of power around 3-4000rpm. I got up the road to an auto shop and replaced the fuel filter, which was absolutely chockers with crap. The problem went away. Then a few hours later it did it again. This time the fuel filter was clean. But after checking it and putting the bonnet down, I got going and it was fine. Then same thing down the road a bit later on, slight hesitation at around 3500rpm. By then I just wanted to get home, so I just kept going and stayed off the accelerator. Pulled up at the servo for a drink for a while. Took off again, problem gone. So it's got me beat. I'm guessing it could be some crap in the bottom of the fuel tank getting caught somewhere? It's definetly not a coil misfire, as I had them all replaced with Splitfire ones which cured that problem straight away. The car runs a 3 inch exaust, an aftermarket fuel pump, 11psi, FMIC and a high flow k&N filter in the stock airbox. Any ideas what might be causing this problem