How to troubleshoot edit node page timeout errors?
I'd guess that a module is performing some task in either form processing or Node API Hooks. It might be communicating with a remote service, or simply taking a long time to perform some action. It might also just be that the DB communication is slow. If such an action takes too long, you'll hit the PHP timeout and the script will terminate.

You could identify which module by several means -


Debugging (eg add debug code with times to module_invoke_all() to see which hooks take a long time)
Understanding the purpose of your modules (I wonder if "Submit To Myspace" module is slowing things down?)
Process of elimination (turn off likely candidates until the problem goes away, turn them back on one by one until it recurs)


Note that the last approach is not the most dependable; if you have a 180s timeout, and module A takes 160s, module B takes 15s, and the rest of Drupal takes 6s, you could easily be mistaken in seeing the issue go away when you disable module B, despite A being the guilty party.

If the issue affects a single page, then sit back and study what might make that specific page different. Eg LinkChecker module (if installed) tries to retrieve linked images or URLs; and if those sites timed out, your node edit might also.