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.