Since you know what a crash sounds like, and aren't hearing that, the most likely failure is somewhere in the read channel electronics. It could be intermittent/poor connections between the pc board and the head assembly contacts. Dunno how desperate you are, but if DW doesn't fix it for you (or makes things worse), you might try removing the drive's pc board, cleaning the contacts and reassembling. I've encountered problems similar to yours a couple of times, and this procedure fixed one (maybe two). In a couple of other cases, a transplant of the pc board fixed it. If you happen to have an identical donor drive (or can find one; a crashed one should be cheap), that might be worth a try, too, if you get desperate enough.

Good luck, and let us know how you make out.