To be honest, every car I've ever had has always seemed to have tracking issues that just won't go away - until I took it to this certain garage who managed to fix it. Unfortunately I've now moved away and said garage is best part of 80 miles away.
- I used to have tracking issues, 20 years ago. If you ask me, those cars had issues with the tracking going out. If you ask the cars, there were probably issues with my driving (although, I ought to mention that there was more 'kerb parking' in where I drove, back then).
- I know you won't think much of this, but if you think that there is a garage which will sort it for you, it might be less hassle just to go there. or, maybe ask locals which companies they rate.
As for tyre pressures, I always do them from cold, with a digital pressure gauge, then double-check the same value shows on the compressor. I *think* I'm on 33PSI all round (whatever info it says inside the fuel filler cap, as that's where I looked to find the right pressures).
So, the car is not a MkIV, then. Well, it might be old enough to have problems with bushes, then.
I doubt that 33 psi all round is what is recommended for 'normal' driving. There are different pressures for different conditions (depending on speed and load) but the pressures for 'normal speeds' (probably under 100 mph) and part loads (different five up and/or with a full load area) are probably lower than that at the rear, and that or a bit higher, at the front. Now, my personal preference is somewhere around the recommended for the fronts and about 2 psi lower for the rear for normal driving, with the fronts up a psi and the rears up closer to the fronts for combined high speed and high load situations (the recommends for the rears change too much, for my money, with load, but that's only my preference...but the rears changing from around 27 psi to around 41 psi is rather too wild for me; maybe if I was in Germany, I'd always go with the high speed recommends).
I can't say that I really like the digital pressure gauges, because the resolution is usually not as good as you'd like. That said, it doesn't sound as if this problem is down to 1 psi measuring uncertainty. In fact, going out on a limb a bit, I'd say that it must be something more dramatic than that. The gauges on the compressors? Now, those can be really inaccurate. I've got a 'stand-alone' analogue gauge, a (Michelin) foot pump and a compressor. The compressor will tell you whether there is air in the tyre, or not, the foot pump gives you a pressure within a couple of psi (the gauge is too small, and any stiction in the mechanism amounts to 2 psi, or so...and Michelin is one of the better ones) and the stand-alone gauge is pretty much spot on when compared against a motorsport gauge, with a calibration certificate (not that I'm actually that bothered, provided that it is consistent and repeatable).
Have you tried doing anything about tyre temperatures? You might not have an optical pyrometer to hand, but it ought to be obvious if one edge of the tyre is running a lot hotter than the rest of the tread. And, you might even get an idea of whether it just happens going round corners, or whether high speeds and straight lines are implicated, too.
I did get a sheet when I had the alignment done, not that I pretend to understand anything that was written on it...
If you got both before and after numbers, one question is whether anything changed dramatically. The obvious things to look for are toe in and camber, but there are probably other important things too, that I haven't quite thought out yet.
I don't think that it can be anything at the back, unless it is way, way out and the front has been set up oddly to compensate. And I can't quite see that happening accidentally, so you'd be primarily looking for big changes at the front.