Bish,
Sounds like your doing it the right way! Just a thought..... before you buy another tire, try switching sides with the front tire & wheel (to check balance only, not perminate), obviously if you can't get it to balance then you know it's a problem on the car, not the tire. Maybe you can narrow things down a bit before spending money.
Of course if you do buy a new tire you can always put the old one in the trunk for a spare.....
Good Luck,
Mike