Trans Slip / 3rd Gear Start

I have had my trans out, replaced seals, modulator, bowden etc. Fluid & Filter changed, fluid at correct level.

When the car is cold, it acts like the trans is slipping, but i really think it's in 3rd or 4th gear. I can manually shift down to first then it's fine. Once the car is warm shifting is normal.

Any ideas what would cause this behavior?

Thanks.