fiesta mk4 scary flat spot at 3500rpm to 5000rpm?

i have a massive flat spot on my fez at mid revs and carnt accelerate out. ive tried checking the usual suspects mfs, iscv, air filter and air flow temp sensor (which has caused problems with low revs) i carnt see it being the lamb sensor as it recently passed on emissions. short of plugging it in at an extortionate rate im out of ideas.