I had this problem (though with some slight variations) last summer here,
and wound up having to replace the alternator/voltage reg. It cleared up
the flickering problem right away. You may want to take the car down to the
local shop and have them check your alternator. I got a beck-arnley repro
here with a lifetime warranty for about $140 (us), and the job is really not
that difficult to do, either, if it turns out to be the alt.
Good luck.