Stellar drift
From The Stargate Omnipedia
A natural phenomena by which stellar matter gradually drifts apart over time as the universe expands outward, eventually making various coordinates invalid. The Stargate networks counter this by automatically dialing one another at regular intervals to update their positions.
These correlative updates allow the Stargate symbols to continually work after hundreds of thousands of years.
KEY EPISODES
Children of the Gods - While examining the Abydos cartouche Samantha Carter and Daniel Jackson realise that stellar drift is why the addresses don't work.
Avenger 2.0 - Samantha Carter speculates the Stargate network undergoes automatic correlative updates to compensate for stellar drift.