Check Engine Light

I have a 1995 Camry with 131,000 miles, and the check engine light comes on and off. The longest it has stayed on is for about 30 min., and the shortest was about 3 mins. It doesn't do it everyday, but about 2 to 3 times a week for the last month and a half. I took it to the mechanic, the ligth wasn't on, but he said to change the catalytic converter, because it looked bad, and that could've been causing it, but about 2 days after the converter was changed it happened again. What could be causing it? There aren't any strange noises from the car, just the light going on and off.