Start with Insp2 service and a tune up. Then wheel alignment. If still bad performance get the injectors cleaned. If yours in a belt drive cam - change the belt if you are doubtful about it's age.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i