Had the same problem (stalling at 25% or 30%).
This worked for me:
1. Reboot computer or use Activity Monitor to force quit any prior Adobe Flash installation process that may be still running
2. Download Adobe Flash update using this link (assuming you are using Safari or Firefox): Flash Player for Safari and Firefox - NPAPI
3. Mount flash player by clicking on flash drive icon on desktop
4. Right click on Flash file
5. Choose 'Show Package Contents'
6. Open 'Contents' then 'Resources'
7. Run file 'Adobe Flash Player.pkg'
8. Reboot computer.