85 discharging

Volt meter shows 11.5 at start up then after 10 minutes or so will go to 12.9 or back and force to 11.8, alternator is ok, put new dist, in new battery, cold this morning was putting out 8.4 volts Tried new alternator same thing what am i missing? Thanks i'm new to this so if i,m doing something wrong bear with me.

After 2 times back to my mechanic with no results went to corvette.com and found 3 problems like mine,simple fix should have thought of it my self, DIRTY BATTERY CABLES s#*t $243.54 later its fine now.