Potomac Electric Power Co. yesterday reported net income of $95.9 million (99 cents a share) for the third quarter ending Sept. 30, up from $86.5 million (89 cents) for the same period last year.

During the quarter, operating revenue was $453.4 million compared with $438.6 million last year, the company reported.

For the first nine months of the year, net income was $176.6 million ($1.80) on revenue of $1.06 billion, down from a profit of $189.5 million ($1.91) on revenue of $1.08 billion in the same period the previous year.

The 1986 per share figures were restated to reflect a 2-for-1 stock split, which became effective May 4.

James Madison Ltd. yesterday reported that net income for the quarter ending Sept. 30 was $627,000 (6 cents), down from $980,000 (15 cents) in the same period of the previous year.

The company said that profit margins were restricted by heavy expenses connected with the opening of the bank's operations center and with costs associated with acquisitions.

For the nine months, profits were $3.4 million (34 cents), compared with $3.2 million (49 cents) for the same period last year.

A.H. Robins Co., operating under Chapter 11 bankruptcy protection because of a crush of Dalkon Shield injury claims, reported that earnings increased 6 percent in the third quarter.

The maker of Robitussin, Chapstick and Dimetapp said net earnings for the third quarter were $26.5 million ($1.10), up from $24.9 million ($1.03).

Sales for the three-month period that ended on Sept. 30 were $229 million, up from $215 million last year.

For the first nine months, earnings were $60.3 million ($2.50), up from $55.6 million ($2.30) last year. Sales were $621 million, compared with $579 million.

A merger between Robins and the Rorer Group of Fort Washington, Pa., is being reviewed by the bankruptcy court.