In testing 63/37 SnPb NC paste I have seen HUGE differences in performance. Have you tried a different paste? I prefer AIM (NC257-2), they have performed well in each round of testing.
My observations regarding mixing old and new paste: The paste has volatile compounds in the mix that eventually evaporate. Mixing old and new will not bring the mix back to new condition regarding these volatile compounds, and printing will suffer. Even new paste has a shelf life. A better question may be to define "old" paste. I have had no problem mixing one-day old paste with new paste, but I do not keep doing this day after day either.