volts drop

does anyone know why my volt gauge goes from 14 to about almost 9 when i put the headlights on along with rear defrost? I have a new alternator and I dont know how old the battery is but I have a battery/load tester and it shows a good battery and full charge on alt. also my jeep has a terrible idle going. it happens when it has warmed up. I replaced the iac but to skeptical to change maps and tb sensor because of the prices on them. also just had a tune up 2 months ago and it started the rough idle around a month after tune up. garage cant find whats causing it. any help would be great.. thanx oh by the way its a 94 jgcl 5.2 143k miles