1998 Buick Riviera Hello

Steering problem
1998 Buick Riviera 6 cyl Front Wheel Drive Automatic 80K miles

I have replaced the rack with new rack,pinion and pump, flush sys. blead thourghly and when engine starts wheels off ground the wheels turn to the left on thier own. When on the ground (wieght of car on wheels) sticks then turns easy... All new parts on front end...

Thanks for any help