Update (to the update...)
OK, so the day after I ordered the new RD, I decided 1 more try on adjusting the current RD. On yesterday morning's ride, the bike performed without any skipping, "soft chunks" (or even "chirps
Initially I thought that maybe I could have held off ordering the new RD...but only for a moment. After 20K miles and 15 years (of questionable maintenance on my part
and the fact that I want to go to 8-spd (which requires a wee bit tighter tolerances) tells me it was time. Yeah, I could have waited. However, with the preciseness which seems to be required on the current RD, I'm sure I'll be better off just going with all new rear chain drive parts.
Again, thanks for listening and for the help. I look forward to more great riding.
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Just wanted to circle back around with an update......
After spending the last couple of weeks trying to find the RD 7th gear "sweet spot" again, I've come to the conclusion that I was lucky up til I cursed myself by threadlocking the RD limit screws. Can't quite get completely rid of the "soft chunks" on flat ground and skipping when going up a hill. And I've pretty much adjusted the high-gear limit thru its range. Also, over the weekend I realized there is just too much play in the bushing(s) (my own doing) where the RD mounts to the the bike (even tho I know these lower end RDs are reasonably forgiving, it escapes me how I ever was able to get it adjusted so well this most recent time). So, as a result, I "pulled the trigger" and ordered up a new RD. Decided on the RD-M410 since I've seen it mentioned as replacement for the original MC-18. The bike's too old to go with a more upscale RD, and the lower-end RDs just seem too cheap.
At the same time, I ordered a new freehub so I can go to an 8-speed cassette. It's not that I want more gears (only use the top 5 anyways). It's that I've noticed the options are ever more limited for 7-gear cassettes. Of course, in a few years, I'm sure I'll say the same about 8-speed, as well
Again, thanks to all for the help....
Safe riding and cheers....Steph