You are not alone Woody. Even after hanging hundreds of doors, it can still drive me mad.
Definitely a case of not being truly vertical. A door may be fitted perfectly, but if the lining (door frame) is not plumb, this will happen.
The trouble is, doors operated in three dimensions. (I know, it's mathematically only two, but you have to think of it in three when trying to compensate.)
Fix one dimension, and the other one will go out of plumb. Togo has made a cracking good attempt at dealing with it, but it really is one of those things that are easily carried out, but almost impossible to describe. You just have to be there.
Anyway, go with bhg. Google "self-closing hinges" or even the old-type "rising butts". (So much in Building sounds porny, doesn't it?)
bhg's Screwfix link is good. Standard size is 3" or 76mm. But you may have 4" (100mm) or even 2 1/2 " (65mm).
I wish I could come and sort your existing hinges for you. It wouldn't take me very long.