It sound like you are having really short total power outs. That can only be bad battery cables/terminations/earth points. cables include this big and small ones. Follow them as far as you can checking for loose or dirty connections. I hope the problem is not behind the dash as you have refitted it twice. If you don't find anything (I think you should, so be very critical) then go back into the dash with special attention on earth points.
The job will take ages (do it over 2 days/weekends so you don't get bored) which is what you would pay shop rates for. You just have to enjoy it - sorry.
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