Jeromie, you are brilliant.
The Apple Forum thread lead me to the solution.
A while back I had experimented with changing the OCSP and CRL settings in...
Keychain Access > Preferences > Certificates
They were not set to "Best Attempt", and reverting them to this setting fixed the installation problem.
All is working now. I am not sure why it was only the Adobe Flash installation that had this problem, but it seems healthy now.
THANK YOU!!