Clean ign switch contacts with contact cleaner, check, clean and tighten every earth point you can find and then look for the others, there are loads of them. Soulds a bit like dirty terminals and contacts to me. OK it might be the module but these other checks are free..
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