Okay. So I've had my 71 charger for some time now and I've had this problem since day one. I've replaced the flasher relay twice. The turn signals will operate normally for a short time then speed up, then slow down, then speed up again to eventually staying on solid. Seems like when I mess with the turn signal switch lever they tend to be a little more normal but not completely normal.
I've checked all bulbs twice, all wiring looks to be good, grounds show no signs of corrosion. Relay could be shot again but I'm wondering if maybe the turn signal switch finally bit the dust? The car does have almost 200k miles on her so maybe it's just a simple signal switch.
When the turn signal circuit speeds up it is losing resistance(load). In your case I think it is something simple. When you look at the bulbs. Did you check the base lead contact points. After time these mush out and can make a poor a connection...Did you make sure the bulb socket contacts are not bent? :Twocents: If this is all good. I would still take the body grounds off and clean them. :Twocents:
From what I've seen all contacts look clean and solid. Body grounds seemed to be okay but that doesn't mean that there isn't any corrosion starting between the body and ground locations.
Try supplying 12v of power just ahead of the flasher (think bypassing the switch itself). You should be able to unplug the flasher and then supply power to one side and plug in the other to the rest of the harness. If you still get a change in the speed, then it should be indicative of changing resistance at the bulb or in the flasher itself.
Terry