Help needed with odd dyno results (2.5l time trial car)

[truegearhead]

I had a power loss problem with the car last season but I thought I resolved it. Instead of the complete power loss I had previously I’m seeing a huge dip in power at 5,000rpm (according to the dyno I’m not feeling it). The air/fuel ratio is a little hard to read and I’m told it wasn’t an accurate reading but all the noise aside it’s definitly leaning out. We ran 5 runs with identical results. Any ideas? It has a brand new MAF and O2 sensors. It’s only throwing codes for rear O2 sensors which are removed along with the cats. The previous shop I took it to is at a loss (not the dyno shop they haven’t looked into it yet).