specifically which time and what EI did you use?
I just had a look at the massive dev chart and found that at EI100, with
stock solution of XTOL, there are times of 6.75 and also 7.5 minutes. I've
never used XTOL but just looking at it's times on various films compared to
my normal developer (LC29 1:19), it generally appears to be a bit slower.
LC29 is supposed to take 7.5 minutes to do TMX, so I'd think that the 7.5
would be the more accurate time. I haven't used TMX either, but with TMZ I
found I always got thin negs using the time listed on digitaltruth, which
was 30sec longer than the time on the bottle of developer.
My conclusion then would be that you need a bit more time in developer.
Alternatively, you didn't say if you were using 1-shot or re-using the
developer - it's not starting to exhaust is it?

First, a disclaimer: I've yet to try XTOL. I've researched it, and I've
got a package of it sitting in my darkroom, but I've not yet used it.

My first thought is that I've heard that TMX users are the most likely to
report instances of "XTOL sudden failure" syndrome, but I've never seen
any reports about whether XTOL that's failed with TMX will work with other
films. Perhaps this is what you're running into, with your XTOL failing on
your TMX but not on your other films. The fact that your XTOL is 8 months
old is at least potentially consistent with this hypothesis. Kodak claims
a 6-month shelf life for XTOL stock solution, and although Kodak seems to
be generally conservative about such claims, it could be you're pushing
it, particularly for TMX and/or if your storage condition or mixing was
less than optimal.

My second thought is that you might be using too high a dilution of XTOL.
Kodak used to provide recommendations for using XTOL at anywhere from
stock strength up to 1:3 dilution, but now they only recommend stock and
1:1 dilution. To further elaborate, Kodak recommends using at least 100ml
of stock solution per 36-exposure roll of 35mm film. I've seen claims that
this recommendation is based on TMX, and that other films can often get by
with greater dilution. Thus, if you're using more than 1:1 dilution (as
some of the DigitalTruth table entries specify), particularly with a tank
that's thrifty with its chemicals, this could be the problem.

FWIW, here are a couple of Web pages with tips on using XTOL:

http://www.covingtoninnovations.com/xtol/
http://canid.com/xtol_faq.html

A Web search on "XTOL sudden failure" will turn up lots more, particularly
related to its propensity for, well, sudden failure. There's been lots
posted on this topic, and everybody seems to have a pet theory about the
cause, but I've yet to see anything that really convincingly nails it
down. I have found a few home-brew formulas that are similar to XTOL with
good-sounding stories about why they should last longer, but little or no
evidence from the field, as it were. If you care to try cooking up your
own developer, check these out:

http://groups-beta.google.com/group/rec.photo.darkroom/msg/a1183c0c58b9b06
http://silvergrain.org/Photo-Tech/film-dev-recommend.html

I've tried the former (but just for a couple of test rolls, so far), but
not (yet) the latter.

   It seems to me that if the developer goes bad it will do
so for all films. From the above it sounds like you have
been getting good results from both films up til now, is
that correct?  Have you developed any more Delta after the
T-Max failure, did they come out OK?
   If you are developing in separate batches is there a
possibility that some error occured, say temperature being
off or the dilution being wrong?
   If you can run a clip test that would tell you if the
problem is the developer stock or something else.
   I have had the Xtol sudden death failure but in my case
the developer didn't develop any film properly.

TMX is particularly sensitive to the amount of Xtol used.  Make sure you are
using at least 100 ml of undiluted Xtol for every 36-exposure roll or
equivalent.  Since you're doing 1:1 that means 200 ml of diluted developer
per roll, which shouldn't be a problem.  From my understanding, TMX is the
reason why Kodak revised their recommendations for dilutions and minimum
developer amounts.

--
Eric
http://canid.com/