Thanks xxnick. That's not a bad idea if there's sufficient wall thickness in the seat tube.As others have said, sounds like theres more to it than the design, as lets face it, loads and loads of bikes have the same set up and dont have this issue.
Sounds like the tubes deformed now after years of overtightening.
If you have a decent bike shop near you it might be worth getting them to check and run a 27.2 reemer down it if needed.
Definitely shouldnt be slipping, and should only need a bit of compression to hold