Internal cable guide gunk

The internal cable guide sheath for the rear shift cable is clogged with teflon from previous inners (i think!). I've tried cleaning and flushing with spray degreaser etc which does flush out a fair amount of the black shavings but once cables are reinstalled the shifting is good for a few rides then quickly goes to censored again. Tried replacing the outers and inners a few times so I'm fairly certain it's the internal run of sheathing. Just wondering if anyone has tips for cleaning it completely or if anyone has managed to run a cable liner through the sheath? Any suggestions or tips would be most welcome!
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