fostex x-12 question

[a27thletter]

what are the best tapes to use in the fostex x 12. i've tried many different brands, all have been high bias type 2, and i've even tried on metal type tape, and they all seem to start degrading very quickly, like, if i record tracks 1 and 2, and play the tape back it sounds fine, but by the time i get to track 3 the tape starts sounding vey warbly, sortof like a tremolo, ,,.. i've cleaned the heads, and i always use the shortest tapes available. i used some 15 minute studio tapes, they worked by far the best, but still seem to degrade way too soon. is there any way to make the X12 record and play back at a higher speed? that would probably help a whole lot if i could.

[a27thletter]

update, i used a head demagnetizer and the recorder works perfect. with the 15 minute studio tapes i get a crystal sound,