Air Carrier departure from BWI voiced concern regarding assigned departure frequencies as related to BWI Tower and PCT Departure Control; alleging multiple events of wrong frequency assignments.

Narrative

I; the First Officer; was the non-flying pilot. Our PDC had a departure frequency listed of 128.7. After takeoff from Runway 15R; BWI Tower switched us to Departure on a new frequency of 119.7. I called Departure and noticed that this frequency of 119.7 was a higher altitude frequency with other communications of aircraft descending out of the flight levels. I switched to our original frequency of 128.7 and Potomac Departure responded with our clearance. I immediately informed him that BWI Tower had given us 119.7 vs. 128.7 to which the Controller responded that they were 'switching frequencies.' The reason for this report is due to a similar problem which I had last month out of a different Washington Metro Area airport. Each problem was associated with 'Potomac Departure Control.' There is a problem in Potomac Approach/Departure. This is my second report in 20 days with communication/frequency issues here. I know for a fact during this incident we did what we were told to do. I am expressing true concern of the ATC system here in DCA.