help w/ dropouts

[tex]

i've got a 424 mk II and i'm in the process of mixing down to stereo cassette -- the problem is, i'm starting to get some intermittent volume drops from the 424 during mixdown on some of the tracks (esp. the drum track). at first i thought the tape was just going bad in spots, but the drops seem to happen in different places every time.

anybody have any idea what's going on? i clean the heads, rollers, etc. regularly and take good care of the tapes. does the head need to be demagnetized? or is the source tape somehow the problem?