I have Shimano 9 speed bar end shifters. A week or so ago the RD shifter came loose and would have fallen out but for the shifter cable securing it. When I got the bike home I took the shifter off and tightened it (to the handlebar) and reassembled it. The next day when I rode I noticed it was not shifting well. In fact it was not only not shifting well it was friction shifting. This bar end can shift SIS or friction, though now it was set SIS mode but friction shifting. I realized that I must have reassembled it incorrectly. So when I got back home I took it apart again and reassembled it correctly.

(Like this https://www.flickr.com/photos/8130268@N04/5589524981)

However, it now shifts erratically. It sometimes slips into another gear when not shifted. The spaces between shift detents does not seem even. I have checked the cable and the housing. The derailleur appears OK. Anyone have any idea what is going on?
At this point I think the shifter is toast.

This might help you.

Park Bar End Shifter Maintenance

Shimano for Dura - Ace Shifter Manual PDF File

A few points which will help improve your situation but maybe not fix all. First I would strongly suggest a coating of grease on all the shifter clamp/expanding elements. I have seen many bar end levers get stuck in bars and the bolt head stripped out because of corrosion and galling. Second- the Shimano end levers have a intermediate piece between the lever and the body which can be placed in various rotational orientations. Only one will work properly, the others will keep the indexed positions from all working. You know about the index/friction choice already. There was a batch of Shimano bar ends that suffered a broken internal part (spring or cover?) which would mess with the lever indexing and returning to the after over shift movement position. It's possible you might have one of these. Andy.

I agree. Also, when tightening the center screw, make sure the "friction/SIS" switch is one way or the other and then really crank the screw down. I've never stripped one, and I've used a broad, T-handle screwdriver on it. This keeps it from loosening and falling between the index and friction positions.

Here's a link to the Shimano PDF that shows the proper orientation of the "lever boss cover"... part #2 on the PDF.
https://bike.shimano.com/media/techdo...9830728256.pdf

Thanks, this helped some.