Odd that it is 1-3-5. My guess is electrical only. Anything else would affect all cylinders. I would also guess not CPS or ICV. Other than that I shall just wait till someone else comes up with the fix and learn something.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i