Hi Lovesbeer:
I happen to know a little bit about the rear sight assembly. The first thing that comes to mind is this.
The rear sight base is staked to the receiver at the arsenal and is near impossible to move left or right without damaging the base. The most notable way the base gets damaged is when someone tries to move the base causing one or both base ears to be bent inward. If this happens, even slightly it will cause the loss of the spring tension that allows the adjustment detent to function properly.
I included a photo of all the small parts in that rear sight assembly for you to see so this makes sense to you because you will need to disassemble the rear sight. You will need a jeweler's screw driver set.
If you look at the windage adjustment knob you will notice 4 notches cut into it on the side that is toward the sight base. Then look at the sping plate next to the knob in the photo. This spring plate is what provides spring tension for the detent to grip the adjustment knob. Notice the spring plate has a very small detent that will correspond to the notches cut in the knob. When things are right the detent on the spring plate catches one of the notches in the knob and that keeps the knob from turning randomly.
If the detent on the spring plate is worn out, or the ears of the base are bent inward or the knob is not fully seated on the screw shaft the detent will not function properly. Notice the indent on the one end of the screw shaft. There is a very small set screw that holds the knob tight onto the shaft. That set screw must be tightened into the indent on the screw shaft in order for the knob to work properly.
With your jeweler's screw driver loosen the set screw within the knob and pull the knob off. Inspect it for wear or damage. If you find the knob not properly installed on the screw shaft this will most likely be your problem. Reinstall the knob and tighten the set screw so it locks into the indent on the screw shaft. If you don't see this as the problem then continue to disassemble the sight. I find a small magnet to be useful to make sure you do not lose these tiny parts. With the knob pulled off you can now remove the spring plate and inspect it to see if the detent is in good shape. If not replace it. I will show you where to get this stuff later. With the knob and spring plate removed you can now unscrew the screw shaft from the sight base at which time the whole thing will fall apart. Be carefull in that there is another tiny coil spring and detent under the sight step that will come out when the sight step is removed. Inspect all parts for wear or damage. Now you will have just the bare base with nothing in it. The 2 vertical ears of the sight base MUST be square and not bent inward in the least. If the ears are bent in you can now take a small Cresent wrench to fit over each ear and gently bend the ears outward so they are once again perpendicular to the bottom of the base.
If you follow these instructions you will find the problem.
Here's a link to Numrich if you need to replace the sight. If you need to replace any or all of the parts inside of the sight base get the new assembly and disassemble it and put the new parts into the sight base already on the receiver. DO NOT TRY TO REMOVE THE SIGHT BASE. You'll be sorry if you try.
Let me know how it comes out. Unless you're a total mechanical retard you will find this very easy.
http://www.gunpartscorp.com/catalog/Detail.aspx?pid=1165850&catid=11905
