What can I do to get my idle normal?

[gqnyc]

I currently got a 1993 Infinti G20...When I start it up my rpm are all good at around 1200-1300 rpm. At times the idle starts jumping/shaking like 800rpm to 775 rpm etc.. When it is in drive and when I hit the brake to stop when im at about at a complete stop the cars rpm drops all way to the bottem I let go of the brake it goes back up and im all good. What should I do to fix this problem clean my throttle body (where is it located) any help would be appreciated

[nametakennow]

Have Autozone or a similar garage check your battery and alternator.