Dancing ammeter

A year and a half ago I rebuilt the wiring harness on my 68 Cuda and upgraded the charging system to 10 ga. wires and also reconfigured it for using the newer style dual field charging system by adding the extra field wire to the new style regulator. I installed a 60 amp alternator, new regulator and new battery at that time also. Everything worked fine for quite awhile but lately I've noticed after you run the car for awhile and the battery comes up to full charge instead of the charging dropping off nice and smooth it starts cycling like your turning the charging system on and off with a switch rapidly. At first I thought the regulator took a crap so I replaced it with a new one and it still does it. No change whatsoever. I took voltage readings at the battery and alternator output with the engine running and there's allmost no difference (maybe .1-.2 volts max). It constantly showed the pulsating on the voltmeter too and made it hard to get a good reading but it was topping off at approx. 15 volts. You can see it in the headlights going down the road. Allmost looks like I'm flashing from dim to brights constantly. Well maybe not quite that bad but you get the idea. You can also see it in the dash lights. I've checked and double checked every ground and connection I can think of. Does anybody else have any ideas?

Again this only happens after the battery has came up to full charge. When I first start it it doesn't do it for a few minutes.

Thanks, Tracy