Replace Stylus or both Cart. & Stylus??

You just need to replace the stylus, which on a Shure is actually an assembly consisting of the stylus, cantilever, magnet and suspension. All that's really in the cartridge body are the pickup coils which should last indefinitely with normal care of the unit.

I don't know if that will necessarily correct the problem you're having though. Perhaps.

Shure says their cartridge shells should last 50 years or a lot longer if cared for. They are after all sealed and have no moving parts. So you should not worry too much about the cartridge.

The only aging issue is the rubber that is part of the stylus assembly. I tends to harden with age if not stored well. If it sounds good to you, maybe it is OK.

The only problem is there are no genuine Shure stylus available anymore from Shure. Most dealers no longer have it. I believe I saw some from LPgear.com that were original (not generic). Also check Needle Doctor. They show them on their website. I have no idea how good the generic replacement stylus are. Some are from Japan, others from Switzerland. The generic replacements probably are just eliptical "E", not HE. But so may be the Shure ones at LPgear and NDoctor assuming they still have them.

There are rubber components in the cartridge assembly that could have dried out over 20 years. I remember they are part of the stylus suspension system, but I don't recall if they are located in the removable stylus piece or if they are located inside the cartridge body. Can you tell by looking at the stylus unit? Obviously, if they are part of the stylus unit, that's all I would replace.

Before you do that, consider that there may be other reasons why the sound quality seems to change with different LPs. For example, you may be aware that some LPs are significantly thicker now than LPs of old, and the difference in thickness results in a change in your vertical tracking angle (VTA). This in turn will affect the sound quality of the playback. You may be hearing that difference, or there may be something else. There might be nothing wrong with your stylus and cartridge. Good luck.