Bicycle Mechanics - Can't keep the Rear Derailer aligned. help!!

It's a XTR on a hardtail mountain bike circa 2000ish. I got it used about a month ago. Sweet ride except for this little PITA issue.

I can get it aligned and shifting great but then after about a 25-30 miles it goes out of alignment again. Suddenly. And by a lot like I lose travel to 2 or 3 rings. Sometimes it's the big rings some times it's the little rings. I'm thinking something must be faulty and needs to be replaced.

Maybe a kinky cable? I can't find any travel problems and the cable looks good on visual inspection. Replacing it is my next plan of attack unless someone here has a better idea.

Maybe a shifter problem? Everything seems ok there. The barrell adjuster isn't loose or broken and the cable is seated properly. Any thoughts?

I can't think of any thing in the derailer itself that could do this.

Someone point me in the right direction ... please!

How old are the shifters? Maybe the rachet mechanism in the shifter might be worn?

I guess that's possible. any test you can think of for that?

Only test I can think of is to eliminate the cable as the possible problem and assume it must be the shifter then. :)

Well, I guess it could be the mechanic...

Check to see if the rear derailleur hanger is bent.

Clean the shifters with a sray degreaser, then relube. This solved a similar problem with thumb shifters for me.

nope hanger is fine.

The popular consensus is that bad cables cause 99% of shifter problems. Complete replacement kits are pretty cheap ($14 - $15) and fairly easy to do. Given the quality of your parts that would probably be the best first move. If it turns out to be something else you may discover it while changing cables and housings.

If it is something else you'll want new cables on the new whatsit anyway. If they are originals (or just old) you won't get the best performance like you will with new cables.

I like your Fountain of Smart. :)

I didn't get time to stop by the bike shop and pick up cable and housing so I just tore everything down, cleaned, lubed and reinstalled. I didn't see any obvious problems but the guide under the bottom bracket was kind of rough. The cable is bare there so perhaps that's where the problem lies. We'll see what happens now.

If it happens again I think I'll replace the cable with "sealed" system - housing all the way from shifter to derailer. What would be the downside in doing this other than weight of the extra housing? Surely someone makes an adaptor to attach housed cables to braze-on cable ends - what are they called? It's much easier on my pride to have a name for the part I'm trying to purchase when I walk in the bike shop. :)

I didn't get time to stop by the bike shop and pick up cable and housing so I just tore everything down, cleaned, lubed and reinstalled. I didn't see any obvious problems but the guide under the bottom bracket was kind of rough. The cable is bare there so perhaps that's where the problem lies. We'll see what happens now.

If it happens again I think I'll replace the cable with "sealed" system - housing all the way from shifter to derailer. What would be the downside in doing this other than weight of the extra housing? Surely someone makes an adaptor to attach housed cables to braze-on cable ends - what are they called? It's much easier on my pride to have a name for the part I'm trying to purchase when I walk in the bike shop. :)

If you're talking about eliminating the sections of exposed cabling, there's probably not much of a problem with just bypassing them and zip tying the housings to the frame, but there's not much advantage either. I doubt that weight is a factor either way. If I were going to modify anything I would probably try to re-route to the top of the BB or higher. I find myself spending too much time with the BB/chainring acting as a skid plate to want a rear DR cable between me and the rock/branch/mud I'm crashing/riding over.

If you get a generic kit it will contain all the cable ends you will need. I bought a Giant brand kit from an LBS and it had all ends, ferrules, cables and housings I needed for the whole job. The cutting of the housings is the only tricky part. Depending on how you cut them off they can get crushed and raggedy. You'll have to re-round them and smooth out the ends (paying special attention to the liner) to get smooth operation. A mechanics class instructor made us all liner tools out of sharpened broken spokes.