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!