lights flicker at cruising rpms fine at idle?

Ok I have 71 dodge demon with a 440....most recently I have changed the alternator (78 amp flat back), it has a newer voltage regulator on it....has new belt, cap and rotor and plugs.

today i was getting ready to hook up the new temperature gauge and got as far as it has the wiring for the lighting hooked up and ran out of time and daylight....I took the car for a drive then noticed that at idle my ammeter sits right in the middle everything is working great but when you go to accelerate or sit at say 2-3k rpms the car appeared to be overcharging and the needle bounced back and forth quite a bit at the +20 to +30 mark

I unhooked the wires to the new gauge and it settled down a little bit but was still bouncing a little bit but staying right around +20....did I mess something up, will it go away when i get the rest of the wires on the gauge?

I plan to check all my grounds tomorrow the best I can just wanted to make sure i did not mess anything up