From the looks of it you were on to a good start by troubleshoting with extensions disabled. It seems to be a Control Panel of Extension that might be causing the freeze up. What was the last thing that was installed on that Mac before it started locking up? If you can answer this, then all you need to do is look for the extension or control panel that was installed by that application, disable it, and then restart the computer with all but the ones you disabled.
If by chance you don't know, then you are going to have to do it one by one. Half of the job is done for you if you select the Mac OS 9 Base option (I believe that's what it's called) under the drop down list in the Extensions Manager. That will only load the control panels and extensions that allow for the OS to function before any third-party extensions/control panels were installed. If it boots OK once you do that, then you can add the third party extensions/control panels one by one until it happens again, which will tell you which is the offending extension/control panel.
Good luck and let us know how it turns out.