Whenever a system goes through changes, there is always a period of time where things are fragile. Any website of a considerable size and complexity will have issues when major updates are done. Some sites have  history and there is sometimes a lot of customization, so changes will have an impact. The best advice I can give is to  take one issue at a time and fix it and the system becomes more and more stable. Nothing ‘randomly’ breaks. Mambo and Joomla are built to handle large sites, and have many interwoven dependencies and variables so you must expect a period of time for flushing out the problems.

Some frustration might come from making changes to the live site. Sometimes, when a site undergoes major changes, a test site is created. Then we make sure the test site is working before switching the domain name to point to the test site. This assumes that the site is thoroughly tested (and sometimes retested after corrections ) by the people that know the site best (the owners)…frontend and back, in the major browsers, etc.

So, for large sites and Joomla, make a test site, and then go at it and test, test, test. Don’t get frustrated unless no progress is being made. And measure your progress by itemizing the issues and checking them off as they are resolved.