Garfinckel, Brooks Brothers, Miller & Rhoades, Inc. yesterday reported a sharp jump in second-quarter fiscal-year sales and profits.

For the quarter ending Aug. 1, the company reported profits of $1.41 million (32 cents a share) compared with fiscal 1980 second-quarter profits of $930,000 (20 cents). Sales rose nearly 20 percent from $97.51 million last year to $116.61 million

In the first half of the fiscal year, Garfinckel's profits rose to $3.32 million (75 cents) from $2.04 million (45 cents) last year, and sales rose 22 percent to $234.27 million from $191.02 million.

The company, which is fighting a takeover bid from Allied Stores Corp., attributed the results to the opening of new stores, improved gross margins and lower expense ratios.

"Our shareholders have yet to realize the full benefit of this program, but we are optimistic that the trend evident in these results will continue," said David R. Waters, chairman of the company.

In other reports, Potomac Electric Power Co. reported seven-month profits of $53.69 million ($1.02 a share) compared with $49.89 million (98 cents) for the same period last year. Revenues rose to $557.18 million from $438.95 million last year.

For the 12 months ending July 31, Pepco reported profits of $107.87 million ($2.13), up from $97.5 million ($1.98) in the previous 12 months. Sales for the period rose to $974.29 million from $777.73 million.

Washington Gas Light Co. also reported sharply higher profits of $18.68 million ($3.51) for the 12 months ended July 31, up from $11.56 million ($2.63) for the previous 12 months. Revenues rose to $509.08 million from $418.45 million.