1995 Other Mitsubishi Models

While idling, my 3000gt seems to have all light dim, and when pressing the gas it comes back to full. The voltage meter in the car reads between 11 and 12 most of the time. And when it rains the wipers slow or stop altogether when idlling. The alternator was replaced at 65,000. Needs a timing belt replaced but other than that there is no problem. What can I do to fix that dimming. Btw, the rest of the electrical in cabin lighs, radio, power windows seem unaffected.