2000 Mitsubishi Galant Disconnected battery = engine stalli

Hi,

I recently was doing some work with my 2000 Mitsubishi Galant and its cigarette lighters. I had to remove the center console to access them, so I disconnected the negative terminal on the battery as indicated by the Chliton manual. When I was finished tightening up the sockets, I reassembled the car and connected the battery. As I drove the car for about 5 minutes, I stopped the car and put it into reverse when the engine died. It starts up as normal again, but anytime I use reverse or turn the air conditioning on-off-on-off it typically kills the engine. When its sitting at idle, the engine kinda "pulsates" and varies its idle speed anywhere from 400-800 rpms, and sometimes makes shuddering feelings when stopped in gear or parked. The engine sounds normal to me, but I am concerned because my Mitsubishi has never had any problems at all until now. I would love to hear what is wrong with the car so I can get it fixed and hopefully enjoy many thousands more carefree miles.