I'm having the same type of problem, except it happens whenever I use more than about 25% throttle and it'll hesitate between 2500 and 4000 rpms. after 4000, all power comes back. my car has an ej20 block with ej22t heads, stock baja turbo, stock wrx downpipe, stock wrx tmic, 4.5 psi boost, ej18 ecu, ej18 220cc injectors, walboro 255 hp fuel pump, and intake. I can't seem to find a single solution to this problem and a couple mechanics I've talked to online, haven't said anything back.So I had a hesitation only when I would accelerate hard between 3-4k RPM then after 4k it would break free and I would get power again. So I thought it could be the cat being clogged. I have no CEL. So I put my new exhaust on, Borla replica header, 2.5" N1 catback with resonator. I also proceeded with my stock cat pipe. So that night I was cruising around, getting on it a little bit still having my hesitation. Then next time I got on it, I had no hesitation! It remained like this for a good 3-4 days. And now just out of the blue I have the same hesitation. I am thinking I have a clogged cat maybe? When I increased the diameter, more air was allowed out the exhuast moving around the broken pieces in the cat and now they have setteled back down and clogged it again?
Any help with this? Only other thought is maybe a TPS or dirty MAF?