are SDLT tapes flaky? 4 of 10 of our tapes are defective

I have a Quantum SDLT320 drive on a linux host for which we've bought: 5
Quantum SDLT-I tapes and 5 HP SDLT-I tapes. I am alarmed that two of each
brand are defective.

Symptoms are twofold:
1 tape took tar files up to (I presume) capacity without error, but "tar
tvf" of those files failed.
3 tapes would take tar files up to ~1/3 capacity (~75GB uncompressed
data), but would not take tar files past that point. mt erase also failed
similarly on those tapes.

I can tar the same data that failed on those defective tapes onto the 6
good tapes. I've tried using other data and retensioning. I've searched
group archives. Is it unusual to get 40% bad tapes from two vendors? (Is
it possible these vendors are producing tapes to 'SDLT160 tolerance
levels' and the tighter write-pattern of SDLT320 can't cope?) Also, are my
verification methods ok: tar to new tape until full, rewind, "tar tvf
/dev/null" and watch for errors. I wonder if it's an out-of-spec drive.
How could I tell? Thanks.