Turn Signal Problem...Different Than Others

Alright, before you fellows jump me with both feet about changing out the board to fix my rear turn signal problem, hear me out. Been there, done that. I have a 96 C3500 that has been plagued with left rear turn signal issues for the last year. I've replaced the board 3 times (T-H-R-E-E TIMES!) and now it's doing it again. Replacing the board fixes the problem...temporarily. Here's what it's doing now: without the headlights on, the turn signals work fine. With the headlights on, the $&$#@ think does the fast click thing. But, when I look at the lights, they are all working...only faster. I don't see any burned out. Now it gets weirder -- I switched the left and right light assemblies and it cures the problem (of course I can't drive it that way or I would in a heartbeat, although duct taping them in place has been a thought). When I switch them back, the problem comes back. If it's a wiring or a board problem, switching the left/right sides should make the problem move around, but it doesn't. Somebody HELP! I would just hang my arm out the window and signal the old fashioned way, but my problem is only at night, so I'm reduced to begging for help.

Just for crap's & giggles, have you swapped out the flasher a hd flasher? There may just be enough resistence in the wiring to freak it out. try it, why not.