well, a bad rectifier and/or battery will NOT affect the turn signals like that. not saying they dont need help, but thats not the turn signal problem.
check the connections to the junction box.. check all the grounds on the bike (ALL of them). check/clean the ignition switch (not the key portion, the actual switch which lives under it).
Also, the easy stuff.. check/replace ALL the bulbs, and look for corrosion in the sockets.
the flasher unit controls both directions of the turn signals with the same pins on it, so if it works right on one, it should be good on both. if in doubt, get a new flasher (recommend electronic one, EP-34, direct swap in, and they are pretty cheap)