I always have to say that I'm a (retired) pilot not a doctor, before going too far with advice, but your particular problem interests me, and there's a lot at stake.
There is just a small chance that your case is caused by a factor other than a God given distortion in the lense/cornea . The variability is the only clue that makes me think that it is worth mentioning the following at all.
By changing angle, I mean that the AXIS changes. They will give a figure like 080 for instance on a script. Now, if this changes quite often, you may be causing the astigmatism, by distorting your eye--by tension for example.
The old fashioned card with black spokes was good at giving a quick indication of astigmatism. Some spokes would be in sharper focus than others.
( by darker I guess they mean a more intense black with sharper edges. )
http://www.eyecareindia.com/subconte...37§ionid=6
Many years ago, my optician (and pal) use to complain that I was never the same twice. Some days there was no indication of astigmatism at all, and it is this similarity, at a not dissimilar age, that makes me think it's worth mentioning.
Once, when the 5 year medical was in London, I had walked down Kingsway against a bitterly cold wind. I could scarcely see the chart, let alone the bottom line. Total panic. The doctor was fine, he said your eyes are in spasm, you'll be okay in a few minutes. He let me warm up, and I was back to reading the bottom line. This was extreme and had a very specific cause of course.
I'm convinced that these spasms can cause all sorts of temporary problems...including slight de-focusing--of one type or another. I am also convinced that anything from cold, to tiredness, or even emotional stress, can be a factor. If you could self check wether, as the day wore on, the angle changed...in a similar pattern each day, this would be a good indication of the processes taking place. I mentioned recently how an LED looks to someone with astigmatism. A small point of light on a hi-fi for instance, may be all you need to get a rough idea. Any astigmatic distortion might look like a secondary lobe, at a given angle from the primary point of light. If this relative position changes with time, you will have a clue.
What to do. You have plenty of time to find out what is going on, but finding a good ophthalmologist must be the way to go. As suggested above, you need to be sure that the readings are accurate. Then you have to explain your concerns so that you get another check a couple of weeks later or so. In fact that would have to be the premise on which you make the appointment.
Should you find that the angle does change a bit, then you need to make careful notes on factors that may be causing the problem, tiredness after long drives, or a stressful day for example. If the error reduced significantly after say, several good nights of sleep, then there is a chance that if you could be relaxed enough for the eye-test, you would not have a problem.