Best guess, after marker pistol grip that the selector spring channel in it is drilled too shallow.


Back off the pistol grip screw by about 4 threads, pull the grip slightly down. and see if that allows the the selector to work normal.
If that does allow the grip to work normal, then break out a drill bit and drill the spring grip channel sightly deepen as needed. Also, when putting the grip back on, make sure that the grip spring channel, and the lower spring channel are maintained so you are not crimping the spring with the grip installed.


If the above does no solve the problem, then pull the grip, catch the spring and detent, and make sure that the detent is moving freely in the receiver channel for it.  The receiver channel is a step channel, so if needed, you can use the drill bits to clean up the the channel so the detent does move freely in the lower receiver.

Click comes from the tip of the detent moving through the selector detent's.

So on that note, check the selector groove detent's to make sure that they are clean (no burs/extra castings), and then dry fit the selector detent into the lower receiver without the selector to confirm that the tip of the detent has a good protrusion into the selector channel to make full contact with the selector groove detents.

If needed and the detent is not protruding into the selector channel (selector not installed), then chase the bottom of the detent step channel in the lower receiver with a 9/64" drill bit to clean the channel and slightly deepen the first step of the  channel so the detent tip can now slightly protrude more into the selector channel.   As for the upper part of the selector detent channel in the lower receiver, it should be cleaned up with a 1/8" drill bit after you have cleaned up the lower channel.

Start off by checking the depth of the detent in the receiver channel for it.  Without a good engagement of the detent to the selector groove slots/dents, you going to have problems either way.  

Once you know that the tip of the detent is protruding into the selector channel (bottom step channel cleaned up), now it a matter of making sure that the grooves/detents in the selector are clean, and  just a matter of matching the tip width of the selector detent to the selector position dents so is the bearing sides of the detent tip seating fulling in the selector dents at fire and safe.  

A quick few swipes of the detent tip across some 400 sand paper will get you started, then just work the selector a few times to further dull the tip of the selector detent to match the selector safe and fire walls detent postions. Granted that you still need the selector detent to have a center tip so it rides the center groove when you turn the selector to hold  the selector in the receiver, it does not need to be so sharp that only the center tip center seats in the selector dents without the sidewalls of the tip firmly touching the selector dents as well.