2008 bmw 528i. Transmission slipping only when it downshifts and 1st gear

With any transmission that has engagement or slipping issues, there is a possibility of electronic concerns within or outside the transmission, as well as mechanical or hydraulic concerns. With this unit, you'll need a BMW compatible diagnostic tool to tell you just about anything. I would suggest visiting a trusted BMW technician near you for guidance. There's simply nothing easily handled on this.