I have the original Shimano 600 Derailleur on my 1997 Specialized M2 Pro.

When changing direction of shifting, it will often not make the shift, especially in the middle of the cluster. For example, starting on the largest cog if I shift up, then up, then up, then down it won't shift on the down. If I shift down again it will move.

Also, sometimes it will shift several minutes after I asked it to.

This has been going on for a while, and seems to be getting worse.

I've adjusted the cable tension multiple times, and sprayed a lot of WD40 and silicone lubricant in there. I've had the RD alignment adjusted by the LBS.

What else should I do?

Thanks!

dirty cables/housing or shifter is gummed up

Another thing to have checked is derailleur hanger alignment.

You may want to check that someone hasn't swapped out your RD if you have a Shimano 600 on it, the Stumpjumper M2 Pro in 1997 came stock with a XTR RD, which would be an M950.

The 600 is a road groupset, and the RD would be shorter than needed for MTB.

If you have just got the numbers wrong, would be looking initally at the cables.

When was the last time you replaced cables and housing? That's the most likely problem.

+1

Delayed upshifting is a clear indicator of sticky cables. The RD spring isn't strong enough to shift against the housing friction, so it can take riding over a few bumps to finally get the RD to move. Odds are this has been going on for a while, getting worse over time. You've probably tried to compensate with trim adjustments so the shifting is now all over the board.

One quick way to confirm a sticky housing is to downshift, then upshift one gear. If the Shift doesn't happen crisply pluck the bare wire away from the downtube like a guitar string. If that completes the shift, then you know it's a cable/housing issue.

Spraying WD-40 isn't going to do it, take it apart flush the housing, spin it dry (outdoors) then grease the wire with a light grease and reassemble. However given the age of your system, I suspect that the only remedy will come with new cables and housings.

+1 My bet is that either the cables and/or housings are worn or dirty or one or more housing strands is coming loose and getting into a ferrule, fouling the shift cable and making it move sluggishly. Replace the cable and housing with good-quality ones, prepare the ends correctly and install the proper ferrules and I bet you will be all set.

M2 Comp was a road bike and came with the 600 group.

In where?

Try spraying a liberal amount of WD40 into the shifters.
If that doesn't work the problem is most likely the cables and cable housings. The short housing found at the rear derailleur is often the culprit because it is subjected to more dirt and dirty spay from the road. Also it is critical to make the short housing the correct length resulting in good alignment where the cable enters the housing at both ends.

Great, I bet that's it (cables and housing). I'll replace them ASAP. Thanks!

Well: New cable, new housing, no grease, slightly bigger loop: Still happens.

Any other ideas? Couldn't the deraiilleur have some gunk in it that makes it sluggish?

Yes, but it's rare and very easy to verify. With the cable off push the derailleur in slightly against the spring and see if it returns crisply. I always double check for a clean shift from the second sprocket to high before attaching the wire, and use that opportunity to fine tune the high gear limit.

What shifters are on this bike? I would open them up and spray the insides liberally with WD40 to clean and flush them out then I would spray a lubricant in there to get them good and free. I know a lot of the Shimano shifters from that vintage have a tendency to bind up and get slow due to drying up grease.

You're probably low on handlebar fluid.

Try spraying some wd-40 into the pivots of the derailleur and in the spring. This may clear up any junk in the derailleur. If that doesn't clear it up, you may need to check the derailleur hanger alignment.

Reminds me of the quote in my signature.

Test with a new RD.

I think I solved it.

I removed the cable and found that the RD moves crisply when pushed with my hand.

I saturated the brifter with WD-40.

I went for a ride, and it was still happening. Then, while riding, I adjusted the tension with the adjuster on the downtube. I don't know if all bikes have this, but I can flick this little lever to one of about five different positions.

For the rest of the ride, every single shift worked perfectly.

So, my conclusion was that I hadn't adjusted things properly, and the RD wasn't lined up just right, so it was somewhat between cogs.

Bravo! Well done. I had a similar problem and during the darkest days it seemed the the gods who have jurisdiction over my corner of the world were all against me. What is a mere mortal to do except persist. Eventually, despite much anguish the problem was solved. I've concluded, that was the test. Again - well done.

Hey! Great you found your problem! One thng that I do that saves a lot of time is to move the rd/fd using just the cable independent of your shifters! Just grab the cable where you can and move the rd/fd in and out just by pulling on the cables. It eliminates guessing that it is your shifters(easier to "work on") rather than that complicated rd/fd! Adjust your rd/fd then work on your shifters! Have fun!