The engine is freshly rebuilt and the tune is also brand new. Originally when I reinstalled the engine the torque converter was causing the car to stall when I came to a stop, the lockup had seized engaged. It still did the hesitation when starting from a stop too, regardless of whether the engine/transmission is hot or cold. I replaced the torque converter with a brand new one and the hesitation still continues, it wasn't cold before I replaced the TC so I can't say if the stalling in reverse is new or not.
It's worth it to say that the stalling in reverse happens only on the first cold start and only happens 1 time. It also happens when I'm not giving the car any gas, since I can simply drift backwards out of my spot. Idle has been bumped to 1000 when in park and 800 when in gear, so idle is plenty high. The car is also fairly heavily modified, it has full bolt ons, a vf52 turbo and forged pistons. Everything is brand new down to the coil packs and spark plugs too.
I have done a log of it when I first start it and I've seen nothing that points to any issues with the tune or engine, I'll capture another one and post it when I can, I'll also do the same but with pulling backwards into my spot so I can pull out forwards.
I do also have a GFB blow off valve, but it's set for full recirculation.
Some further information, the hesitation definitely feels like something is physically holding the car back, like if you leave the parking brake on by accident. Is it possible that it has something to do with the brake band in the transmission? I know these can get worn down pretty badly.