Could just be worn parts when fitted? If you can get hold of a reference book of them (parts places used to have books, maybe still do?) you can sometimes find other shafts the same dimensions but can cope with more torque. You should be fine with the correct bits though unless you have more power going through them, or your driving style is perhaps a little too rough? Try easing off the power gradually (but still quickly) in a smooth motion as opposed to just lifting off (dunno if you do "enthusiastic" driving round town or whatever, lots of traffic light GP's, rapid start/ stops while being too rough). If it's just normal use on a normal car then something is at fault somewhere in the drivetrain, be it worn parts, loosely fitted parts (eg. allowing play from the box- shaft from loose multispline bolts) or something else?
If you are fitting 2nd hand shafts then its worth pulling the boots back and filling the joints with fresh grease, look for swarf or other crap too. While you're there grab the shaft with one hand and the cup with the other and try and rotate them in opposite directions back and forth to see if there's play. Change the boots too if they're worn as you can fail MOT on this now I think, not to mention having to do it again when it splits/ leaks anyway.
