Do you know how much older the Sigma is in relation with the 5?
It is known that older 3rd party lenses had AF problems with newer AF systems from Canon and Minolta.
A few years ago I used to see advertised lenses from Sigma, Tokina, etc. that worked only on early Minolta bodies.
The Dynax 7000i is a very nice camera. Pity all those cards, but it turns out to be at times a fun thing.
As your Sigma works well with the 7000i, then just accept the 5 is a tad too modern for it.
There should be no problems between older Minolta AF lenses and newer bodies.
The reason why older 3rd parties don't work with newer bodies is due to the chip not being compatible. Canon and Minolta (at least these 2, there might be others) didn't fully release the specs for their AF mount to 3rd parties. So, a bit of reverse engineering was needed.
In 2002 I bought a shiny new Sigma EX 70-200mm HSM for my then new EOS 3. Even though booth were bought new and made in that same year, the Sigma needed to go back to manufacturer to be re-chipped as it didn't work with the EOS3.
One thing you could try though: with another lens try to activate only the central AF point and swap with the Sigma. See if that works.