I'd make a few suggestions.  Please note I'm not medically qualified
so you should not take this as medical advice, always consult your
doctor before taking any action.

1. Chronic stuffy nose.  This is often a sign of house dust mite (HDM)
allergy, or it could be that you are still affected by cat allergen.
I recommend you get another set of allergy tests now, as you may have
additional allergies to those that were detected as a child.  I believe
the best test is a blood test (RAST test) which examines your blood for
specific allergen antibodies, although a "skin prick" test will give a
good first approximation.  People with cat allergy are very often allergic
to HDM and grass pollen as well.  I wouldn't rely on allergy tests that
were taken more than a few years ago, the picture can change.

2. If you have HDM allergy, you need to remove the HDM habitats from your
house.  To do that, you need to:
- remove all soft furnishings, especialy cloth-upholstered furniture, and
replace with hard versions or plasic/leather-covered (but watch out for hidden
cloth surfaces in plastic/leather covered suites that can vent the allergen);
- remove all carpets and replace with a hard floor; carpets are a health risk;
- cover all bedding mattresses with mite allergen barrier covers, ditto
for pillows and comforters; wash any blankets weekly at >60%C;
- routinely open the windows to reduce indoor humidity and ventilate house;
- remove all fabric curtains, replace with plastic curtains or roller blinds;
- remove all soft toys; don't bother trying freezing them, just throw them out;
- throw all the old clothes in the wardrobe/cupboards in the trash;
- minimise the number of books, magazines and papers lying around the place;
they all collect dust;
- vacuum inside your PC now and again; I get loads of dust collecting
inside mine, and the case fan blows it straight out into the room;
- perform all vacuuming with a HEPA cleaner (but be aware that many
manufacturers now use "HEPA" as a marketing term while their cleaners
still leak allergen from the machine).

This all sounds fairly radical, but actually it's nowhere near as bad as it
sounds once you start doing it, you soon get used to living in the more
healthy home environment.

To have a clinical impact on allergic symptoms, you need to reduce the
allergen levels by at least two orders of magnitude; it's no good doing
this by half measures.  The bedroom is the most important place to reduce
your allergen exposure.

Having said the above, clinical trials of allergen avoidance have
typically shown disappointing results, due to the near-impossibility
or eliminating HDM populations from modern homes.  However, reducing your
allergen load can't possibly do any harm.

Clinical trials of room air filters have shown that they have no
value in improving patient symptoms.

The UK national asthma campaign site www.asthma.org.uk gives a good
rundown of what allergen avoidance techniques are valuable, based upon
the current scientific thinking.  Another good site is A.A.I.R
http://www.users.globalnet.co.uk/~aair/.

3. As you know you have cat allergy, do you know if there have ever been
cats in the house, before you moved there?  The cat allergen chemical
is non biodegradable (ie it stays active for decades, as does the
HDM allergen) and adheres to all the surfaces in the house, eg walls,
ceilings, carpets, curtains etc (the submicron sized particles are normally
in suspension in the air when cats are in residence and some adhere
electrostatically to the interior of the building).  Even if there has not
been a cat in the house for years, you could still be exposed to the allergen.  
You only need to breathe in micrograms of the allergen to get an allergic
reaction.  

If there has even been a cat in the house, you need to steam clean or wash
all surfaces to remove any traces of the allergen.  Avoid cats like the plague.

4. Pollen allergy is another potent source of chronic stuffy nose.  Pollen
levels are usually highest at night resulting in the common experience of
waking up with a blocked nose.  

If your new allergy tests show pollen allergy, you could try window pollen
filters or local steroids (eg nasal fluticasone) or mast-cell inhibitor
eye-drops, along with the antihistamines.  Maybe your central air
conditioning filter would take the pollen out, although I'd be prepared to
bet it's not as good as the manufacturers claim.

5. Do you live near busy roads which carry lots of traffic?
Diesel engines emit submicron carbon particles (PM10's) which are highly
dangerous, they cause lung inflammation and long term lung disease as
they are of a size that lodges in the lung and is not expelled with the
breath, once they're in there, you're stuck with them.

In the UK there has been at least one PM10 "national incident" this year
where the government air pollution monitoring service has detected dangerous
levels of PM10's in the air over much of the country.  Counter-intuitively the
PM10 levels can be higher in rural areas than in cities, basically underneath
the downwind city pollution fallout zone.  The increasing popularity of
diesel cars means that PM10 emissions are going up.  If you live near
lots of traffic, I'd move.

Hope some of this helps.

-- gumbo