Maxcapacity

I've been having this problem for over 2 weeks and I just cant figure it out. The car drives perfectly fine until I want to use the power, between 3/4 and full throttle in vtec it feels like its going good then all of a sudden it feels like instantly letting off the gas when I'm not.
The car has been tuned to 15psi on my built block, ran great for 6000km then I had this problem.
No codes other than the normal code 20 and 36 I had before when it ran fine.
I have gone over every ground, timing is good, dist., plugs and wires are brand new gap set at .028 no loose connections I am just stumped
Has anyone had this similar prob. with a solution?
Thanx