One or more of the fuel injectors may short out electrically causing a rough idle condition.Google+
Car Problem Reports
Chevrolet Beretta Fuel injectors can short-circuit
Chevrolet Beretta Problem
Engine Affected: 3.1L V6
Average Mileage: 101,913 mi (91,118 mi - 112,708 mi)
Visitor, 1991 Chevrolet Beretta, 3.1L V6, 112,708 mi
I dont know how many of the fuel injectors broke but I had to replace all 6.