Do cartridges "dry" out?

I have 3 really nice sonus cartridges with low hours. Although they sound great, after several hours of use, they seem to get "tired" or "weak" sounding. If I stop an clean them throroughly they better. When fresh and clean, they blow away my Blue Point and Shure.
Someone told me that the parts in an older cartridge will dry out? Is that true? I never had this problem before with my pre-amp and don't think it's the problem.
Classe 30 pre with phono
Sota Jewel table with Jelco LMT arm
Sonus Bronze, gold and white cartridges.
Elevick, the short answer to your question is yep. But there's a cheap fix, popular with reviewers in the 80s from Stereophile and The Absolute Sound. Buy some Stylast stylus protector from a high-end shop (not the Last cleaner, but the Last "Stylast", if it still exists), load up the little brush with the fluid, turn your cartridge upside-down, and rub the brush on the cantilever gently so you see one drop of the Stylast disappear into the cartridge. Leave it a few minutes, then re-install and start to play it. One hour later, the bass and dynamics will re-appear (this really works). Now, some people refuse to try it because of unknown effects in the long-term. If you are not planning to send it for a rebuild at a cost of several hundreds of dollars (at least), then this simple process will allow you to enjoy your cartridge for years to come. All things eventually die, so enjoy and don't worry! Of course, if it is a true classic which should be kept pristine, then spend the money and send it for a rebuild. I did this to a tired Kiseki Blue three years ago, and it is still the most reliable cartridge I have, always there when another of my cartridges crap out.
I recall the Sonus cartridges were mid-priced moving magnets from the 80s. Suspension hardening generally occurs over the long term (years) not short term (hours) so I'm not sure what is causing your observations. Also, suspension problems seem to be more of an issue with moving coils than moving magnets (maybe because no-body keeps their moving magnet stylii long enough for them to harden). Sounds like dirt on the stylus but this should be heard as audible distortion. I guess you could try the Stylast treatment on your cheapest cartridge. If that works, what-the-heck, dunk them all in it!

Phono cartridges use an elastomer (rubbery stuff) to mount the stylus in a compliant manner. All elastomers harden up with age. Sometimes (often) they can be softened up again by treatment with a fluid. This is how the car automatic transmission "tune up" products work. They soften the seals that have become hard and leaky with age.

So there is good technical reason to believe that Stylast would work. Auto transmission "tune up" stuff would probably work just as well, at a fraction of the cost. Put a drop in your cartridge and the rest in your car.