Years ago I had a new service drop installed and the conductor was copper. The electrician who did the work used a TAP made for aluminum service drop. I had much the same problem you are having.
In short, I found this out by calling Philadelphia Electric who replaced the tap with one made to interface their Aluminum line with my copper and the problem went away. The line men told me that this sort of things happens all the time.
Oh yea, No charge from PECO.
