It sounds like the little ball bearing or spring on the ball bearing under the collar is missing, if you have ever diss-assembled a lens, its an easy thing to find, but finding the correct parts to fix it could be a hassle. If you are only going to use it in "A" mode you could try to lock it in place. Generally its a small 1mm spring and ball bearing on most lenses that gives that "click" feel, if your confindent, its not that hard to pull off the aperature collar and find if its missing.
I have run a strip of cardboard under an aperature collar to stiffen up the friction on the collar.....
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