At what point does DSTV Signal Quality % affect picture quality?

Perhaps a stupid question but on some days, HD picture quality on the HPVR is not great with ghosting and artifacts (not to mention SD programmes) - if I then check Signal Status then I get say, 83% Signal Strength and 90% Signal Quality

My installer assures me this Signal Status is ok - but when I get 100% Signal Quality (which happens) then my picture (HD and SD) suddenly looks much better.

So my questions are:
1) Does picture quality degrade if Signal Quality drops and when (at what Signal Quality percentage) should one expect visible deterioation?
2) How steady can one expect Signal quality to be?

As indicated I do get 100% Signal quality but sometimes it drops and I can't work out if it is weather related or what.

The HD channels also sem to vary in quality - some are amazing and others are very dissappointing.

1. No, up to a point though.
2. 80%+

I think your mind is paying tricks on you. The stream is digital so as long as it's being received without errors your picture stays the same. If things deteriorate you will see a blocky artifacts or no picure at all.

Not all channels are the same. Depends on the amount of bandwidth dstv allocates to it. Also some HD shows will differ as you have mentioned. One would think it would be the same accross the board but in my experience it can vary quite a lot